Triethylenemelamine (TEM) has been tested for use as an orally administered positive control mutagen in somatic cell cytogenetic assays in rats. The overall occurrence of abnormal cells per total cells analyzed per group given 2.0, 1.0, or 0.5 mg/kg of TEM was significantly greater than negative control values; however, variability of individual response was such that the average incidence of abnormal cells per rat did not differ significantly from negative controls. Such variation within a group was not observed when TEM was administered intraperitoneally or intramuscularly. Because of this variability, triethylenemelamine administered orally was unsatisfactory as a positive control compound for cytogenetic tests of mutagenicity in rats.