An allogeneic effect factor (AEF) derived from mixed lymphocyte reaction (MLR) cultures of alloactivated A.SW (H-2s) responder T cells and irradiated A/WySn (H-2a) stimulator spleen cells helps an in vitro primary anti-erythrocyte plaque-forming cell PFC response of BALB/c nude spleen cels and also A/WySn but not A.SW T cell-depleted spleen cells. AEF activity is adsorbed by anti-Ik and anti-I-Ak but not by anti-I-Jk, anti-I-ECk, and anti-Is. Gel filtration of ACA 54 resolves AEF into two main components that which appear in the 50,000- to 70,000-mol wt (component I) and 30,000- to 35,000-mol wt (component II) regions, respectively. Component I has a mol wt of 68,000, elutes from DEAE-Sephacel at 0.05-0.1 M NaCl, and has an isoelectric point (pI) of 5.8. It helps A/WySn but not A.SW B cells and, therefore, is H-2 restricted. Component II is not H-2 restricted, because it helps both A.SW and A/WySn B cells. It also stimulates (a) the growth of a long-term cytotoxic cell line in vitro, (b) Con A-induced thymocyte mitogenesis, and (c) the generation of cytotoxic T cells. The latter three properties of component II are not shared by component I. In addition, component II elutes from DEAE-Sephacel at 0.15-0.2 M NaCl and has a pI of 4.3 and 4.9. Ia determinants and Ig VH, CH, L-chain, and idiotypic determinants are not present on either component I or component II. The properties of component II are identical to that of a T cell growth factor produced by Con A-stimulated spleen cells. It is suggested that the H-2-restricted component I of AEF might be an MLR-activated responder T cell-derived Ia alloantigen receptor.