CHO cells were grown for 36 h in the presence of BUDR. 12 h before harvesting, the cells were irradiated with UV or with fluorescent (FL) light. Part of the cultures were treated with cysteamine (Cys) during irradiation. Metaphase spreads were stained by the Hoechst-plus-Giemsa method, and the frequency of SCEs per chromosome was determined in second-mitosis cells. The basal frequency of SCEs (1.07 +/- 0.073) was not increased by Cys treatment (1.15 +/- 0.048). On the other hand, UV and FL induced a 3-4-fold increase in the frequency of exchanges (4.18 +/- 0.2 and 3.32 +/- 0.12 resp.). Cys, when present during irradiation, markedly reduced the frequency of UV- and FL-induced SCEs (2.7 +/- 0.14 and 2.29 +/- 0.11 resp.). It is assumed that Cys prevents the formation of radiation-induced breakages in the BUDR-substituted DNA. The residual increase over the basal levels of SCEs which remains after the treatment with Cys may be due to the persistence of some open breaks, to the presence of protein-DNA crosslinking, or to the combined action of both types of lesion.