Parent leads to F1 chimeras were prepared by reconstituting sublethally irradiated H-2 heterozygous mice with marrow cells from one parental strain. Purified parental strain T cells prepared from unprimed chimeras were exposed to sheep erythrocytes in heavily irradiated mice of each of the two parental strains and recovered from thoracic duct lymph of the recipients at either day 1 or day 5 posttransfer. The lymphoborne cells were then tested for their capacity to collaborate in vivo with B cells of the two parental strains. From this approach it was concluded that parent leads to F1 chimera T cells contain two discrete subgroups of T-helper cells, one specific for self H-2 determinants and the other restricted to H-2 determinants of the opposite parental strain. The restrictions mapped to the K-end of the H- 2 complex.