Using the polymerase chain reaction we have isolated and sequenced cDNA clones corresponding to the polymorphic first domain of the DR beta 1 chain from the DR4, "Dw13" cell line, JHa. We have found that the JHa DR beta 1 allele differs from previously reported Dw13 alleles by a single amino acid substitution at position 86. The functional relevance of this polymorphism is supported by the reactivity pattern of a T-cell clone, E38. E38 is an alloreactive T-cell clone which reacts with all Dw14 stimulator cells and all Dw13-positive cells tested except the "Dw13"-positive homozygous typing cell line JHa. Inhibition studies with monoclonal antibodies revealed the stimulating determinant to be on DR and not on DQ or DP molecules. These data indicate that position 86 of the DR beta 1 chain can play an important role in the formation of determinants recognized by T cells.