Partial albinism, an autosomal dominant defect of dorsal skin with primarily ventral albinism, was observed in a family of six generations. There were 24 affected members, three of whom also had heterochromia. Electron-microscope studies demonstrated the absence of melanocytes and presence of Langerhans' cells in the albinotic skin, an increase of melanin granules in the hyperpigmented borders, and normal histologic appearance of the normally pigmented skin. These features could be explained either by incomplete migration of melanoblasts to the ventral midline, or by a defect in differentiation of ventral melanoblasts. The presence of similar electron-microscope findings in acquired vitiligo, the probable relationship of Langerhans' cells to melanocytes, and the transplantation studies of piebald spotting in mice all tend to favor a genetic defect in melanoblast differentiation.