We have demonstrated that human lymphocytes can respond to the synthetic polypeptide GLPhe upon in vitro challenge by the antigen similar to that of (H,G)-A--L, (T,G)-A--L, (Phe,G)-A--L, and GAT. Family studies further support our postulation that responses to these synthetic polymers are under dual gene control. Three families with intra-HLA-A/B recombinants provided mapping information for Ir-GLPhe genes. The response phenotype of the recombinant of family 21 localized the Ir-GLPhe genes toward the HLA-B of D regions, whereas recombinants of family 24 and 27 placed the Ir-GLPhe genes distal to HLA-B, toward the A region. This discrepant gene assignment can be explained by assuming that recombination occurred at different positions between HLA-A and HLA-B. In family 21, crossover occurred distal to the Ir genes, while for the other two, proximal to them. A second possibility is that as in the mouse the two complementing genes are situated in different regions of the human major histocompatibility complex (MHC) and all three of the crossovers occurred between them with the putative Ir-GLPhe-1 located near the HLA-A region and Ir-GLPhe-2 on the HLA-D region or vice versa. A third possibility is that immune response required interaction between a complete HLA-D-like molecule encoded in the A region and another encoded elsewhere, perhaps in HLA-D.