In an attempt to determine the factors involved in ocular pigment variation, the eyes of pallid mice (C57BL/6J–Pa/Pa), and those of littermate black mice (C57BL/6J–+/Pa) were examined ultrastructurally, histochemically, and biochemically. As controls, the eyes of black and congenic albino mice (C57BL/6J +/C2Jand–C2J/C2J) were also examined. In the developing pallid mouse eye the retinal pigment epithelium (RPE) and choroid contained immature melanosomes with incomplete melanization. In the adult pallid RPE, numerous lysosomal structures containing melanin components were present, while in choroid, melanosome numbers not only increased but many were aggregated in large membrane–bound granules. These RPE and choroidal structures may be melanolysosomes because of their histochemical acid phosphatase positive reactions. Biochemically, the activity of acid phosphatase in ocular homogenates of pallid eyes was significantly higher than in either the black eyes of +/Pa or in the albino and black eyes of the C2Jgenotypes.Dopa reactions at the light and electron microscopic levels indicated that Dopa oxidase activity was present in the cells of the RPE and choroid of both pallid and black mice. Histochemically the Dopa reaction of pallid pigment cells seemed to be equal to that of black ones, suggesting that tyrosinase production might be normal in pallid pigment cells. Biochemically, however, tyrosinase activity in the pallid eye was significantly lower than in the black control.It is concluded that the active digestion of melanosomes, as well as the low ocular tyrosinase activity and the immaturity of pallid melanosomes, may contribute to the pigment dilution observed in the pallid mouse eye.