The genotoxicity of effluents collected from a conventional 5-stage softwood kraft pulp bleaching process was studied in Chinese hamster ovary (CHO) cells in vitro. Spent liquor from the first chlorination stage (C/D), where elemental chlorine and chlorine dioxide had been used in equal proportions, was shown to induce a dose-dependent increase in sister chromatid exchanges (SCEs) without metabolic activation (4-h treatment), with a maximum increase of 1.6 times over the control level at 204 microliters/ml; this dose also induced 15.5- and 20.5-fold increases in cells with chromatid-type chromosomal aberrations after 4-h and a 20-h treatment, respectively. Another C/D stage spent liquor from a process where the ratio of elemental chlorine and chlorine dioxide had been 9:1 produced a 40.5-fold elevation of cells with chromatid-type aberrations at 204 microliters/ml (20-h treatment). This sample clearly increased chromosomal aberrations also when tested as a concentrate (4-h treatment), which showed that the observed clastogenicity was not unspecifically due to the relatively large volumes used in the treatments with the unconcentrated liquors. In general, the use of rat liver S9 mix reduced the genotoxicity of the spent liquors. The results agree with earlier findings on the Salmonella mutagenicity of the same C/D samples: both the prokaryotic and eukaryotic assays showed a reduction in genotoxicity when the amount of elemental chlorine in the bleaching process was reduced. An effluent sample collected from the alkaline stage of the process was not clastogenic with or without metabolic activation. Methanesulfonyl chloride, a new compound identified in bleaching plant air, was found to be induce chromosomal aberrations in the presence of S9 mix.