C4 is composed of two tightly linked genes (C4A and C4B) lying within the major histocompatibility complex of chromosome 6 that can be demonstrated by agarose gel electrophoresis. Seven alleles and five alleles at the C4A and C4B loci, respectively, were detected in 169 black individuals from the southeastern United States. Furthermore, the phenotypic frequencies of C4A6, C4A5, C4A4, C4B4, C4B3 and C4BQ0 were significantly different between black and white Americans.