Chinese hamster ovary (CHO) cells were treated with neocarzinostatin (NCS) and analyzed for chromosomal aberrations and sister chromatid exchanges (SCE). After treatment the cells were recovered for 9, 20, 26 or 30 h. NCS induces chromosomal aberrations and SCE. SCE were much more frequent in cells with chromosome type aberrations at 20 h recovery time than in those with chromatid type aberrations at 9 h recovery time. In second post-treatment cells at 26 or 30 h recovery time NCS induced chromosomal aberrations but only few SCE.