A number of novel cyclic amidine analogs of chlorambucil were synthesized and examined for cytotoxicity in breast cancer cell cultures and for inhibition of topoisomerases I and II. Evaluation of the cytotoxicity of these compounds employing a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay and inhibition of [(3)H]-thymidine incorporation into DNA in both MDA-MB-231 and MCF-7 breast cancer cells demonstrated that these compounds were more active than chlorambucil. The degree to which these compounds inhibited cell growth breast cancer cells was directly correlated to DNA-binding affinity. These studies indicate that cyclic amidine analogs of chlorambucil are a potent catalytic inhibitor of topoisomerase II but not topoisomerase I. The highest degree of DNA binding and cytotoxicity in both MDA-MB-231 and MCF-7 breast cancer cells was observed for the compound, which possess a 4,5-dihydro-1H-imidazol moiety.