The levels of tyrosinase activities, tyrosinase cross-reacting material (TY-CRM), and tyrosinase mRNA were determined for skins from five congenic strains of mice (Mus musculus) with different coat colors. The dopa oxidase activity and melanin formation activity were directly proportional to the abundance of TY-CRM in all of the mutants. The levels of the enzyme activities and TY-CRM were increased in brown (b/b) and dilute (d/d) mice, while they were reduced in agouti (A/A) and pink-eyed dilution (p/p) animals. In albino (c/c) mice, the tyrosinase activities and melanin formation activity were scarcely detectable, and no TY-CRM was observed. Contrary to these, the levels of tyrosinase mRNA in agouti and albino mutants were almost the same as that in black mice, and those in brown and dilution mutants were 20-30% higher than that in black mice. These results indicate that the mutations at the brown and dilute loci exert their influence on the tyrosinase mRNA level, presumably by affecting the transcription of the tyrosinase gene, and that the mutations at the agouti and albino loci exert their influence on the TY-CRM level by regulating translation of tyrosinase mRNA and/or post-translation modification of the enzyme.