B (Tyrp1 ), the wild type allele at the mouse brown locus, produces black eumelanin, while b (Tyrp1^b), the recessive allele, produces brown eumelanin and exhibits lower tyrosinase (Tyr)-related protein 1 (Tyrp1) activity. However, it is unknown whether melanocyte proliferation and differentiation are affected by the Tyrp1^b mutation. The proliferation rate of brown (C57BL/10JHir (B10)-Tyrp1^b/ Tyrp1^b) melanocytes cultured in a serum-free melanocyte proliferation medium (MDMD) was similar to that of black (B10-Tyrp1 /Tyrp1 ) melanocytes. Although brown melanocytes exhibited normal morphology, their pigmentation was lower than that of black melanocytes. However, Tyr activity in brown melanocytes was increased both in vivo and in vitro. Melanosomes of cultured brown melanocytes were mostly spherical stage III melanosomes with granular depositions of pigments, whereas those of cultured black melanocytes were mostly stage IV ellipsoidal melanosomes with pigment depositions in intraluminal fibrils. Chemical analysis of melanin present in dorsal hairs of 5-week-old mice from the F₂ generation between brown and recessive yellow (B10-Mc1r^e/Mc1r^e) or agouti (B10-A/A) mice showed that eumelanin content was greatly decreased in brown and brown agouti (cinnamon) mice, whereas pheomelanin contents in brown recessive yellow and cinnamon mice did not differ from the corresponding Tyrp1 /- mice. These results suggest that the brown allele greatly inhibits eumelanin, but not pheomelanin synthesis.