Erythrocyte and tissue isozymes of human AHCY have been studied by starch gel electrophoresis, cellulose acetate electrophoresis, isoelectric focusing and Na dodecyl sulphate electrophoresis. The same isozyme was observed in all the tissues studied, suggesting that human AHCY is encoded by a single structural locus. Two variant alleles were identified in erythrocyte AHCY using starch gel electrophoresis in a sample of 166 unrelated individuals from the British population. The gene frequencies were 0.024 for AHCY*2 and 0.006 for AHCY*3. The variant isozyme patterns could not be distinguished by isoelectric focusing. Using the homologous rat cDNA AHCY probe, human AHCY cDNA recombinants were isolated from a placental cDNA library. The human and rat sequences show considerable homology in the coding region of the gene and also, but to a lesser extent, in the distal part of the 3' untranslated region. Preliminary observations suggest the occurrence of a high frequency PvuII site RFLP identified with the human AHCY probe.