A C4 cDNA clone has been used to localize the C4 genes within the human major histocompatibility complex (MHC). The clone, pC4AL1, detects a DNA polymorphism of the C4 genes in Southern blot hybridization experiments and established the genotype of a lymphoblastoid cell line, LCL 721, as heterozygous for the C4 DNA patterns. Subclones of LCL 721 that had gamma-ray-induced lesions of the MHC were analyzed with pC4AL1. Loss or retention of chromosome-specific C4 DNA patterns relative to the loss or retention of other MHC genes on the same chromosome was assessed. This permitted the mapping of the C4 genes, and the other MHC-linked complement genes, between the HLA-DR and HLA-B loci.