Dominant lethal and heritable translocation studies were performed in male mice receiving a single intraperitoneal injection of trimethyl phosphate (TMP). The germ cell stage investigated was the spermatid. Methyl methanesulfonate (MMS) was used as a positive control in the latter study. A dominant lethal assay gave marked dose-dependent increases in early fetal deaths. Heritable translocations were detected at 1000 or 1500 mg of TMP/kg in F1 male progeny when screening for semi-sterility and cytogenetically analyzing the meiotic or mitotic chromosomes. Translocation induction was higher at the higher TMP dose (14.3%) than at the lower dose (5.3%) and the yield from the higher dose was similar to that induced by 50 mg of MMS/kg (11.0%). Most of the translocation carriers were semi-sterile or sterile. The data confirm conclusions from other dominant lethal studies showing TMP to be capable of causing chromosomal damage in mouse spermatids and show that certain types of damage result in heritable translocations.