Two loci, C4A and C4B, of complement C4 are located in the class III gene region in human MHC. C4A and C4B allotypes are autosomal codominant by the mendelian inheritance. Twelve C4A and twenty C4B allotypes and null allele at each locus have been reported. C4A locus has been found to be duplicated in some haplotypes. The phenotyping of C4 was performed on carboxypeptidase B and neuraminidase treated EDTA-plasma samples from 117 unrelated healthy Koreans. The samples were subjected to high voltage agarose gel electrophoresis followed by immunofixation with anti-C4 antibody. C4B allotypes were further detected by a haemolytic overlay method employing sensitized sheep erythrocytes and C4-deficient guinea pig serum. Four C4A and four C4B common alleles and two rare C4B allales were observed. Allele frequencies were as follows: C4A^*2, 0.022; A^*3, 0.700; A^*4, 0.133; A^*QO, 0.145; C4B^*1, 0.633; B^*2, 0.214; B*3, 0.008; B^*4, 0.004; B^*5, 0.103; B^*QO, 0.038. C4A^*3 and C4B 1 were the most common alleles. C4A^*1 and A^*6 were not detected in these samples, but those alleles are relatively common in Caucasian (0.010 and 0.035) and Black American (0.057 and 0.057). B^*QO was relatively lower than other populations. B^*5 frequency was similar to that of Japanese, which is rare in Caucasians and Black American. B^*5 appears to be characteristic allele of Oriental and some C4 allotypes show a remarkable difference in different ethnic groups.