Data on 15 polymorphic protein-coding loci are used to estimate the proportion of Caucasian genes in U.S. blacks from the greater-metropolitan area of Pittsburgh. Allele frequencies from U.S. whites of the same region and from a sample of Nigerians are used as representatives of the genetic contributions of the source populations between which admixture has occurred. These materials provide 18 unique variants that occur exclusively in the blacks and 5 variants that are restricted to the Caucasians only. As a result, when all segregating alleles (52) at these 15 loci are considered, the proportion (mean +/- SE) of Caucasian genes in U.S. blacks (25.2% +/- 2.7%) is estimated with a precision much better than that of all other previous estimates. The estimate based on the frequencies of these 18 unique variants of African origin (24.8% +/- 6.2%) is also consistent with the pooled estimate obtained from the 15 loci by the weighted least-square method. The homogeneity of locus-specific estimates of admixture indicates that these loci are appropriate for studying the proportion of black genes in any admixed population involving African admixture. The advantages of employing such loci for genetic-epidemiologic studies in U.S. blacks is discussed in the context of the availability of these large number of unique African variants at these protein loci.