Thiopurine methyltransferase (TPMT) catalyzes the S-methylation of aromatic and heterocyclic sulfhydryl compounds such as the drug 6-mercaptopurine. In humans, TPMT activity is inherited as a monogenic trait. It would be useful if there were an animal model in which the genetic regulation of TPMT could be studied. Average TPMT activities in livers of C57BL/6J (B6) and AKR/J (AK) mice were only 17 to 29% of average activities in livers of like-sexed DBA/2J (D2) mice. Average TPMT activities in kidneys of B6 and AK mice were only 41 to 45% of average activities in kidneys of like-sexed D2 mice. Breeding experiments were performed to study the possible role of inheritance in regulating variations in TPMT activity in these mice. TPMT activities in livers and kidneys of F1 (hybrid) animals (N = 38) from D2 X B6 matings were intermediate to those in the parental strains but were closer to D2 than to B6 values, an observation that suggested partial dominance of the D2 phenotype. The results of studies of F2 (N = 107) and backcross (N = 102) animals derived from these matings were compatible with autosomal recessive inheritance of the trait of low TPMT activity in these mouse strains. Of the F2 animals, 27.1% were included in a "low" TPMT subgroup when enzyme activities in livers and kidneys were both used for phenotypic classification. TPMT activities in livers and kidneys of F1 mice (N = 40) from D2 X AK matings were also intermediate to those in the parental strains but were closer to D2 than to AK values.(ABSTRACT TRUNCATED AT 250 WORDS)