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As mononuclear phagocytes have been implicated as important cellular elements in the process of bone resorption, we decided to study the relevancy of macrophage (M phi) activities to bone resorption. In this study, we investigated the phagocytic activity and activities of lysosomal enzymes of peritoneal resident M phi from rats fed a high-sucrose diet (Diet 2000) to appreciate the effects of Diet 2000 on systemic and local factors. Minkin et al. have postulated that bone-derived chemotactic factors were released from foci undergoing resorption. And so, we examined the effects of the supernatant from alveolar bone cultures (Bone-sup) prepared from rats fed Diet 2000 on the activities of glycogen induced peritoneal M phi. As a result we observed mild alveolar bone resorption with slight inflammation when the rats were fed Diet 2000 for six months. In the periodontal tissue, we found inflammatory cell infiltration, destruction of the periodontal ligament, and lacunae in the alveolar bone due to resorption. The phagocytic activity of M phi treated with Bone-sups was suppressed before the periodontal tissue, which is inflammatory condition such as alveolar bone resorption. Furthermore the phagocytic activity of resident M phi taken from rats on the Diet 2000 was suppressed. After one month of the Diet 2000, the activity of acid phosphatase (AcP), a lysosomal enzyme of M phi, was suppressed, but by six months it was enhanced. The activity of beta-N-acetyl-D-glucosaminidase (NAG), another lysosomal enzyme of M phi, was suppressed over the total period of Diet 2000 before the periodontal tissue was destroyed. These findings suggest that the capacity for defense against infection by M phi is suppressed when periodontitis is initiated by Diet 2000 feeding and that M phi activities are influenced by some factors elaborated by cells in the alveolar bone.