Variation in sensitivity to cisplatin during the cell cycle was studied in synchronous CHO cells treated during G1 or late S phase. The cells were assayed for cell killing, cell-cycle delay, and chromosomal aberrations after they were treated with cisplatin (1-12 micrograms/ml) for 1 h at 37 degrees C. They were either plated for colony survival, or colcemid was added from 12 to 40 h after plating followed by fixation 4 h later for analysis of chromosomal aberrations after the cells completed 1 or 2 cycles (i.e. first or second mitosis). Cells treated with 6 micrograms/ml exhibited about a 10-h delay during the first cycle after treatment during G1 compared with about 3 h during the first cycle and 6 h during the second cycle after treatment during late S. In both cases, cells entering metaphase exhibited predominantly chromatid-type breaks and exchanges. For both cell killing and chromosomal aberrations, the cells in G1 were 1.5-1.6 times more sensitive than those treated in late S, with 1 aberration per cell corresponding to about 37% survival. However, the exchanges and breaks were observed primarily in the first mitosis when cells were treated in G1 compared with the second mitosis when cells were treated in late S. These results suggest that DNA replication opposite cisplatin cross-links in the DNA results in lethal chromosomal aberrations.