Fifty-three donors belonging to seven families were tested for their immune response potential to (H,G)-AL. Most of these donors had been previously tested for their ability to respond to (T,G)-AL and were all HLA typed as well. The heredity of the ability to respond to (H,G)-AL by the production of an antigen-specific helper T cell factor is compatible with an autosomal dominant trait linked to HLA. The genotype of an HLA-A/B recombinant individual suggested that a gene controlling the immune response to (H,G)-AL is linked to HLA-A. Lod scores also suggested a linkage between immune response potential to (H,G)-AL and HLA-A. The different patterns of responses to (T,G)-AL and (H,G)-AL observed in many individuals are compatible with the notion that separateloci are governing the immune responses to the two synthetic polypeptides.