Molecular polymorphism of the B complex was studied in serologically defined B19 haplotypes by use of class I, class II, and class IV probes in Southern blot experiments in chickens. All chickens studied shared identical class IV restriction patterns. In contrast, class I and class II probes revealed six and five subtypes of B19 haplotype, respectively. These subtypes may be resolved in three homozygous genotypes and their corresponding heterozygous combinations. Previous genetic selection allowed us to distinguish two subpopulations in these B19 chickens with regard to the fate of Rous sarcoma virus (RSV)-induced tumors. Molecular genotyping was applied to B19 chickens challenged with RSV in order to determine whether there is a correlation between one of the molecularly defined subtypes and the progressor/regressor phenotypes of the chickens. None of the molecularly defined subtypes correlated with the progressor or regressor phenotype of the challenged birds.