We determined that myeloma cells which survived drug treatment had an altered sensitivity to cytotoxic antibody. The effects of several chemotherapeutic agents differing in drug action were compared. The antiserum was directed against viral determinants on the surface of S107 myeloma cells. This antiserum was then used to inhibit the colony formation of drug-treated myeloma cells. Tumor cells were treated with melphalan (200 ng/ml), methotrexate (40 ng/ml), actinomycin D (5 ng/ml), or 0.5 mM thymidine for 24 hr and then washed and resuspended in fresh medium. On various days after drug treatment, aliquots of these cells were exposed to complement and dilutions of antiserum and then cloned in soft agar in order to quantify the degree of antibody-mediated inhibition. Melphalan, methotrexate, and thymidine caused a severalfold increased resistance of the tumor cells to the antiserum during the first 1 or 2 days after drug treatment. During the following days, however, myeloma cells showed markedly increased susceptibility to the antiserum. The biphasic effect of methotrexate or thymidine treatment was similar to that previously observed after melphalan treatment and differed from the effect of actinomycin D. Actinomycin D caused only an increased susceptibility of the tumor cells to the antibody for a period of 4 to 5 days immediately following drug treatment. Our studies indicate that several chemotherapeutic drugs at concentrations comparable to those used in humans have long-lasting antagonistic as well as synergistic effects on the sensitivity of tumor cells to antibody and complement, depending on the particular drug used and on the time interval between drug exposure and subsequent treatment with antibody. These results suggest a model for evaluating the use of antibody in the elimination of malignant cells which, despite exposure to chemotherapy, remain clonogenic and proliferative.