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The expression of antigenic determinants, Rg/Ch/WH, on Japanese C4 allotypes has been studied. Although the Japanese C4 allotype frequencies are known to differ from Europeans, the antigenic expression of their C4 allotypes correlates with associations described previously. All 89 random donors and 17 selected donors were Rg:1,2 so neither Rg:1,-2 nor Rg:1,-2 was found. The frequency of Ch:1,-2,3 was elevated while that of Ch:1,2,3 was reduced, which was seen as a direct result of the higher frequency of B2 and B5 allotypes. None of the Japanese were Ch:1,2,-3, but this can be accounted for by the absence of the A*6,B*1 haplotype. The WH determinant, which has been associated completely with Rg:1,-2 in Caucasians, was found at a higher frequency, 32%, in association with an A*3,2,B*QO haplotype expressing Rg:1,2, which has not been described previously. Detailed investigation showed that the A3 allotype was Rg:1,2 whereas the A2 allotype only expressed Rg1 (Rg:1,-2 WH+).