Previous studies have shown a synergistic interaction between trastuzumab (Herceptin; Genentech, Inc, South San Francisco, CA) and the cytotoxic drug cisplatin in human breast cancer cells. To define the nature of the interaction between trastuzumab and other classes of cytotoxic drugs, we applied multiple drug effect/combination index isobologram analysis to a variety of chemotherapeutic drug/trastuzumab combinations in vitro. Synergistic interactions at clinically relevant drug concentrations were observed for trastuzumab in combination with cisplatin, docetaxel, thiotepa, 4-OH cyclophosphamide, vinorelbine, and etoposide. Additive cytotoxic effects were observed with trastuzumab plus doxorubicin, paclitaxel, methotrexate, and vinblastine. One drug, 5-fluorouracil was found to be antagonistic with trastuzumab in vitro. In vivo drug/trastuzumab studies were conducted with HER-2/neu-transfected MCF7 human breast cancer xenografts in athymic mice. Combinations of trastuzumab plus cisplatin, docetaxel, cyclophosphamide, doxorubicin, paclitaxel, methotrexate, etoposide, and vinblastine in vivo resulted in a significant reduction in xenograft volume compared to chemotherapy-alone controls (P < .05). The synergistic interaction of trastuzumab with specific chemotherapeutic agents suggests rational combinations for testing in human clinical trials.