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Background: To explore the safety and utility of combining low dose single‐agent doxorubicin with a canine specific anti‐CD20 monoclonal antibody (1E4‐cIgGB) in client owned dogs with untreated B‐cell lymphoma. Animals: Forty‐two client‐owned dogs with untreated B‐cell lymphoma. Methods: A prospective, single arm, open label clinical trial of dogs with B‐cell lymphoma were enrolled to receive 1E4‐cIgGB and doxorubicin in addition to 1 of 3 immunomodulatory regimens. B‐cell depletion was monitored by flow cytometry performed on peripheral blood samples at each visit. Results: Dogs demonstrated a statistically significant depletion in CD21+ B‐cells 7 days following the first antibody infusion (median fraction of baseline at 7 days = 0.04, P <.01) that persisted throughout treatment (median fraction of baseline at 21 days = 0.01, P <.01) whereas CD5+ T‐cells remained unchanged (median fraction of baseline at 7 days = 1.05, P =.88; median fraction of baselie at 7 days = 0.79, P =.42; Figure 1; Supplemental Table 3). Recovery of B‐cells was delayed, with at Day 196, only 6/17 dogs (35%) remaining on the study had CD21+ counts >0.5 of baseline, indicating sustained B cell depletion at 4+ months after the final treatment. 1E4‐cIgGB was well tolerated with only 1 dog exhibiting a hypersensitivity event within minutes of the last antibody infusion. Conclusions: The canine 1E4‐cIgGB anti‐CD20 monoclonal antibody is apparently safe when administered with doxorubicin and effectively depletes B‐cells in dogs with DLBCL. [ABSTRACT FROM AUTHOR]