In contrast with earlier reports on the induction of sister-chromatid exchanges (SCEs) by SH compounds in cell lines of the Chinese hamster, cysteine, cysteamine and cystamine did not cause an increase of the SCE frequency in human lymphocyte cultures. Differences in the treatment protocols or variations of the BrdUrd concentration had no effect on the induction of SCEs by these substances. The inclusion of H2O2 and comparative investigations with V79 cells of the Chinese hamster showed that the probable reason for the SCE induction by SH compounds is the inability of the cells to degrade H2O2. Furthermore, for cystamine it became clear that additional effects must exist besides the induction of SCEs through H2O2. The present study underlines the fact that the examination of a substance within one cell system does not necessarily permit a reliable statement about the DNA-damaging property of this substance.