In this experiment 40 adult male C57BL mice were used. The one ear of each animal was irradiated with high pressure mercury lamp (Burdick'UV-800) and the other ear was covered with nontransparent tape. Using split epidermal sheets treated with DOPA solution, a quatitiative study of epidermal melanocyte was carried out from the exposed and the shielded ear continuously on every second day during 14 daily exposures to 100 mJ/cm of UVB. The results were as follows : 1. Repeated radiation with ultraviolet light elicited a significant increase in population of melanocytes and the possible mechanism is a direct replication of the functioning melanocytes. 2. UVB irradiation induced increase in the number of epidermal melanocytes in covered ear as well as in irradiated ear. It is suggested that the population increase in the shielded skin is initiated by one or more systemic factors originating from the UVB irradiated skin.