The chemotherapeutic agent busulfan was tested for the induction of dominant lethal and specific-locus mutations in male mice. A dose of 5 mg/kg b.w. of busulfan induces dominant lethal mutations in spermatozoa. A dose of 20 mg/kg b.w. induces dominant lethal mutations in spermatozoa and spermatids. A total of 83,196 offspring were scored in the specific-locus experiments. Busulfan-induced specific-locus mutations were recovered in spermatozoa and spermatids, but not in spermatogonia. The sensitivity patterns for the induction of dominant lethal and specific-locus mutations by busulfan in germ cells of male mice are similar but not identical.