Studies with H-2-congenic and recombinant strains showed that when F1 hybrid T cells were activated to sheep erythrocytes in irradiated mice of parental strain or related strain, a population of helper cells was generated which collaborated only with B cells sharing the K-end of the H-2 complex with the strain used for activation. No evidence was found that the restriction in helper function (a) reflected a deficiency of appropriate macrophages during T-B collaboration, or (b) was influenced by the Ig allotype of the B cells. It was concluded that the results signified restrictions acting at both the level of helper cell induction (presumed to be a reflection of T-macrophage interactions in the irradiated intermediate hosts) and during T-B collaboration. With (CBA X C57BL/6)F1 cells, the restrictions at each level mapped to the same region i.e. to the left of the I-B subregion. Consequently, one gene (or set of genes) might control restriction at both levels. If so, T-cell recognition of major histocompatibility complex-associated antigen on macrophages and on specific B cells would be either identical or very similar. The fact that genes mapping to the K-end of the H-2 complex also control the restrictive interactions of homozygous T cells implies that F1 T cells behave functionally as a mixture of T cells derived from the two parental strains. Positive selection to antigen in parental strain mice appears simply to alter the ratio of these two populations.