The candidate antigen for Vogt-Koyanagi-Harada's (VKH) disease was isolated from cultured human melanoma cells (G-361). First, the soluble protein from the melanoma cells was separated into several fractions according to the molecular weight by gel filtration. In each fraction, the antigenicity was analysed by lymphocyte proliferation assay using peripheral blood lymphocytes (PBLs) from VKH disease patients. Then the positive fractions were further separated by liquid isoelectrofocusing followed by SDS polyacrylamide gel electrophoresis. In the liquid isoelectrofocusing fractions, the fraction at about pH 6.0 contained the antigenic protein. The protein from the 75 kDa band on the SDS-PAGE of this fraction was able to cause proliferation of the PBLs from the VKH disease patients. Thus, it appears that the 75 kDa protein of pH 6.0 isoelectrofocusing point was the specific autoimmune antigen for VKH disease.