1,5,2,4-dioxadithiocane-2,2,4,4-tetraoxide (compound II) and 1,5,2,4-dioxadithionane-2,2,4,4-tetraoxide (compound III) are two structural analogues of the anticancer agent cyclodisone. Compound III was found to be more toxic to human colon carcinoma cells of the Mer- phenotype (BE) than the Mer+ phenotype (HT-29). In contrast, compound II showed little preferential toxicity with both cell lines being equally sensitive to this agent. DNA interstrand crosslinks were induced in the sensitive cell line by compound III but only at concentrations which were extremely cytotoxic. No DNA interstrand crosslinks were detected in the resistant cell line by compound III nor in either cell line by compound II. Total crosslinks which reflects both DNA interstrand and DNA-protein crosslinks, were observed in both cell lines after exposure to each agent although the kinetics of formation and repair of these lesions differed between both the compounds and each cell line. DNA strand breaks were also induced in both cell lines by each compound and were found to be totally 'protein associated' with compound III and to a lesser extend with compound II. The mechanism of action of this class of compound is thus different from other bifunctional alkylating agents and has still to be identified.