Helper cell induction to nonparticle antigens in vitro requires the cooperation of T cells and macrophages, but does not occur if the macrophages are allogenic. The reasons for this were investigated. Malfunction of allogenic macrophages was excluded by cultures with their syngenic T cells; suppressor cell induction was excluded by admixture experiments. Thus, T cells and macrophages only cooperated if they were genetically similar. The genetic locus (loci) involved was mapped. Using congenic lines differing only at the H-2 complex, the genetic control of T-macrophage interaction was localized in the H-2 region. Mice with intra H-2 recombinants were used to map the T-macrophage interaction locus in the I-A region of the H-2 complex (formerly known as poly-D, L-ala-poly-L-lys. Recombinants were also used to exclude the presence of another T-macrophage locus either the K, I-B, or I-C, SS-Slp, or D regions of the H-2 complex. Genetic restrictions for T-macrophage interaction in helper cell induction was shown in mice of the H-2-k, d, b, q, s genotypes as well as in H-2 recombinants. The possible mechanisms and significance of this genetic restriction are discussed.