Monnclonal antibody HMB-45 was previously thought to be melanoma specific antibody and was taken recently as a marker for melanocyte proliferation or activation. A single UV-irradiation system which may induce a functional activity change in melanocytes was used to test the irradiated epidermal melanocyte for expression of HMB-45.Melanocytes, in the epidermis of the biopsies taken 7 days after a single UV exposure, showed markedly stained HMB-45,accompanied by morphological changes such as enlarged size,rich cytoplasma and elongated dendrites. We conclude that since morphological changes in irradiated melanocytes may be interpreted as signs of functional activity changes, melanocytes are exposed to sthnulatory factors induced by UV light and get into a functional activating state to produce melanosome and HMB-45 antigen. Besides, the presence of HMB-45 in a single UV exposed epidermal melanocyte may support the hypothesis that its expression is inducible and is closely related to the functional activity The Monnclonal antibody HMB-45 was previously thought to be melanoma specific antibody and was taken as a marker for melanocyte proliferation or activation. A single UV-irradiation system which may induce a functional activity change in melanocytes was used to test the irradiated epidermal melanocyte for Expression of HMB-45.Melanocytes, in the epidermis of the biopsies taken 7 days after a single UV exposure, with markedly stained HMB-45, appended by morphological changes such as enlarged size,rich cytoplasma and elongated dendrites. We conclude that since morphological Changes in irradiated melanocytes may be interpret as signs of functional activity changes, melanocytes are exposed to sthnulatory factors induced by UV light and get into a functional activating state to produce melanosome and HMB-45 antigen. Also, the presence of HMB-45 in a Single UV exposed epidermal melanocyte may support the hypothesis that its expression is inducible and is closely related to the f Unctional activity