Abstract
Three partially inbred substrains of Obese strain chickens were studied for the spontaneous development of autoimmune thyroiditis. The influence of the major histocompatibility complex (B haplotype) was marked, less marked but still significant and barely detectable and transient in the 3 substrains, respectively. These differences in the effect of B haplotype may be due to the overriding action of genes other than those within the B haplotype.