SUMMARYLymph nodes, spleen, and thymus (but not bone marrow) contain alloantigensensitive units (AASU) which proliferate in the spleens and lymph nodes of lethally irradiated, allogeneic recipient mice. Proliferation was assayed by measuring the incorporation of 5-iodo-2'-deoxyuridine-125I (125IUdR), a specific deoxyribonucleic acid (DNA) precursor. Exposure of grafted lymphoid cells to H-2 or multiple non-H-2 alloantigens was necessary to induce proliferation. Growth was exponential between 2 and 6 days after grafting, and linear relationships existed between inoculum size and125IUdR uptake. No evidence of nonspecific stimulation of lymphoid cells or of marrowthymus cell synergism was detected. Alloantigenic stimulation enhanced generation of AASU by their immediate precursors, under conditions of limited pool size. Thymectomy prevented differentiation of AASU from marrow stem cells but did not affect formation of AASU from immediate precursors or terminal differentiation and proliferation of AASU. Mutant alleles at theWlocus did not affect the growth of AASU, suggesting that separate genetic controls exist for proliferation of AASU and myelopoietic progenitor cells. However, the same immunogenetic controls affected transplantability of AASU and myelopoietic progenitor cells, since both hybrid histocompatibility(Hh-1)and modifying “resistance” genes were operating in irradiated recipients of lymph node and bone marrow cell grafts.