Ultrastructural studies of the effects of the chemotherapeutic agent platinum-thymine, on the morphology of sarcoma-180 ascites cells were studied to elucidate the cancer cells immediate response to therapy and the possible mode of cancer cell regression. Sarcoma-180 ascites cells treated with platinum-thymine in vitro at concentrations of 60 micrograms/ml at varying time intervals demonstrated that the drug is capable of killing cancer cells. The cells exhibited drastic nuclear and cytoplasmic alterations with few lipid spherules in the cytoplasm. The cell volume increased tremendously, with the nucleus relatively larger than the cytoplasm as time of treatment increased. Degradative features of the nucleus, and especially the nucleolus, inhibited metabolic processes. Nucleolar and cytoplasmic necrosis with increased cell volume and vacuolation are potential signals for cancer cells response to the drug and a possible mode of causing the eventual demise of the cancer cells.