The C4 and 21-OH loci of the class III HLA have been studied by specific DNA probes and the restriction enzyme Taq 1 in 24 unrelated Sardinian individuals selected from completely HLA-typed families. All 24 individuals had the HLA extended haplotype A30,Cw5,B18, BfF1,DR3,DRw52,DQw2, named "Sardinian" in the present paper because of its frequency of 15% in the Sardinian population. Eighteen of these were homozygous for the entire haplotype, and six were heterozygous at the A locus and blank (or homozygous) at all the other loci. In all completely homozygous cells and in four heterozygous cells at the A locus, the restriction fragments of the 21-OHA (3.2 kb) and C4B (5.8 kb or 5.4 kb) genes were absent, and the fragments of the C4A (7.0 kb) and 21-OHB (3.7 kb) genes were present. It is suggested that the "Sardinian" haplotype is an ancestral haplotype without duplication of the C4 and 21-OH genes, practically always identical in its structure, also in unrelated individuals. The diversity of this haplotype in the class III region (about 30 kb less) may be at least partially responsible for its misalignment with most haplotypes, which have duplicated C4 and 21-OH genes, and therefore also for its decreased probability to recombine. This can help explain its high stability and frequency in the Sardinian population. The same conclusion can be suggested for the Caucasian extended haplotype A1,B8,DR3 that always seems to lack the C4A and 21-OHA genes.