Chinese hamster ovary (CHO) cells were treated in the G1 phase of the cell cycle with different concentrations of neocarzinostatin (NCS) alone or in combination with N-(2-mercaptopropionyl)-glycine (thiopronin; TP). TP reduces the frequencies of NCS-induced chromosomal aberrations (CA) and of sister chromatid exchanges (SCE) significantly when added to the cultures simultaneously (TPsim), 1 min (TP1) or 10 min (TP10) after the addition of NCS. The addition of TP 30 min (TP30) or 60 min (TP60) after NCS reduces the frequencies of SCE, but not of CA. Our results indicate that the induction of CA and SCE by NCS is partially based on different mechanisms.