The molecular basis for the genetic control of variable proportions of the two cat hemoglobins Hb A(alpha 2 beta A2) and Hb B (alpha 2 beta B2) was investigated. Ratios of Hb A/Hb B vary between 50/50 and 90/10 among members of the mongrel cat population, with clusters around 50, 35, and 10% Hb B. Genomic DNA from cats of 50/50, 70/30, and 90/10 phenotypes were cut by restriction endonucleases HindIII, EcoRI, BamHI, BglII, and Pstl and hybridized to a fragment of the human beta-globin gene. The results of the Southern blots suggested a pattern of homozygote, heterozygote, homozygote for the respective cat phenotypes, 50/50, 70/30, and 90/10. Therefore, the cat hemoglobin polymorphism seems to result from the possible combinations of an allelic gene pair.