Introduction A private label (PL) is defined as a brand owned, sold and distinguished by retailers (Lincoln & Thomassen, 2009). Therefore, most PLs display only their brand name on their product labels or packages. However, in the Japanese consumer goods market, an increasing number of manufacturers’ names are now being displayed on PL product packages. For example, the “Seven Premium” PL, by Seven & I Holdings, displays the manufacturer’s name on its product labels using the phrase “This product is a joint development product with manufacturer X.” This indicates that retailers are utilizing the brand of the national brand (NB). This type of branding strategy can be classified as co-branding or a brand alliance. In Japan, expansion of PL co-branding may have improved consumers’ attitudes toward PLs and contributed to their development. Theoretical Background Most prior research on co-branding has focused on brand alliances between two NBs. Therefore, studies on alliances between PLs and NBs are very limited. Vaidyanathan and Aggarwal (2000) focused on ingredient branding, which merged elements of PLs with NB ingredients. Based on the combination theory (Park, Jun, & Shocker 1996), attitude accessibility theory (Fazio, 1986), and attribution theory (Heider, 1958; Kelly, 1973), they found that the association of brand name ingredients with private brand products could positively impact consumers’ evaluations of unfamiliar products. Also, the use of a brand name ingredient in a PL did not negatively affect consumers’ evaluations of this product. Arnett, Laverie, and Wilcox (2010) focused on brand alliances of retailers and manufacturers in the clothing category. Based on the attitude accessibility theory and the information integration theory (Anderson, 1971; Smith, 1993), they found that consumers’ attitudes toward alliances can influence retailer equity, manufacturer brand equity, and shopping intentions. They also found that pre-alliance retailer equity significantly affects attitudes toward alliances. Except for post-alliance retailer equity, perceived fit moderates all relationships between consumers’ attitudes toward the outcome of an alliance. Prior research on co-branding indicates that the attitudes and attributes of co-brands are influenced by the attitudes and attributes of constituent brands. In addition, a constituent brand’s familiarity moderates the effect (Park et al., 1996; Simonin & Ruth, 1998). As another notable finding, attitudes about co-brands influence constituent brand attitudes, with the effect being moderated by brand familiarity (Simonin & Ruth, 1998). Based on prior findings, this study examines whether co-branded PLs are influenced by their constituent NBs and by the familiarity of constituent brands in the case of Japanese co-branded PLs. This study also examines whether a spillover effect exists for constituent brands. Methodology An Internet survey of 798 women living in the Greater Tokyo area was conducted in August 2017. Subjects were assigned to one of four groups, grouped by PL and NB brand familiarity (Figure 1). The familiar “salad dressing” category was selected. While this category includes several large and popular manufacturers, many small, unknown manufacturers also exist within it, a fact that is relevant to the purpose of this study. High-familiarity PLs and NBs were selected based on their market shares in the category. However, Seven Premium, the most popular PL in Japan, was not selected as it had already adopted a co-branding strategy. Aeon’s Topvalu was chosen as a high-familiarity PL. While Topvalu is one of the most popular PLs in Japan, it has not yet taken a co-branding strategy. All measures were assessed through a seven-point, semantic, differential scale. Attitudes toward constituent brands (PLs and NBs), co-brands, perceived quality of constituent brands, and brand familiarity of constituent brands were measured. Using covariance structure analysis, we examined factors influencing attitudes about co-branding and the spillover to constituent brands after considering co-branded formations. Regarding familiarity, a multi-group analysis was conducted. Results Attitudes toward constituent brands (PLs and NBs) positively influenced attitude toward co-branded PLs. In addition, the influence on the co-branded PLs was greater for PLs (Table 1). From the multi-group analysis, the influence of familiarity on the attitude toward co-branded PLs can be found in some cases (Table 2). In the case of high-familiarity PLs with high-familiarity NBs (Group 1), the attitudes about co-branding by NBs were not significant. On the other hand, a positive co-branding attitude by NB was found in other cases. In cases of low-familiarity PLs with low-familiarity NBs (Group 4), the co-brand’s influence was greater for NBs. Moreover, since the attitude toward co-branding positively influenced differences of attitude between post-alliance and pre-alliance toward PLs and NBs, the spillover effect is confirmed (Table 3). Discussion Our results, which suggest that co-branding with NBs can be an effective strategy if PLs have low familiarity, are consistent with prior research. PLs in Japan, which have a lower penetration ratio than those in Europe and the United States, are still in a developmental stage (Kumar & Steenkamp, 2007). Since PLs in Japan are less familiar than leading NBs, co-branding with an NB can be an effective strategy for a PL. On the other hand, when a PL is already established as a brand and has high familiarity, co-branding with a NB might have little effect. In this case, it might be necessary to devise a different strategy, such as changing a PL’s brand name. Seven Premium, the most popular PL in Japan, was introduced in 2007. From the beginning, when its brand had low familiarity, it opted for a co-branding strategy, and our findings suggest that this decision significantly improved overall attitudes toward PLs.