Fresh human mononuclear cells (MNL) cultured in serum-free medium and stimulated with concanavalin A (Con A) produce a lymphokine functionally similar to the previously described antigen-induced lymphokine human lymphocyte mitogenic factor (LMF). T lymphocytes appear to be the source of Con A-induced LMF and the presence of adherent monocytes is required for maximal production. The proliferative response to LMF, assayed either as supernatant fluids in the presence of alpha-methyl-D-mannoside or as partially purified 27,000 to 35,000 m.w. fractions from Bio-Gel P-100 columns, indicates LMF is distinct from Con A. LMF acts almost exclusively on B lymphocyte-enriched (E rosette-negative) MNL while exhibiting only minimal activity on T lymphocytes and no activity on monocytes. LMF preparations induce both B cell proliferation and the maturation and proliferation of E rosette-negative T cell progenitors into E rosette-positive cells. Assay of interleukin 1 (IL 1) and interleukin 2 (IL 2) preparations for LMF activity implicate IL 2 as the T cell progenitor stimulating factor. Assay of the proliferation of human IL 2-dependent alloreactive T lymphocyte line/ATLL) indicates that LMF preparations contain IL 2, IL 2 activity, however, is completely removed by absorption of LMF preparations by ATLL cells, whereas LMF activity (B cell proliferation) is unabsorbable, thus distinguishing LMF from IL 2. These absorbed LMF preparations contain no IL 1 activity as assayed by murine thymocyte proliferation, nor does IL 1 have LMF activity. These results indicate that Con A-induced LMF contains a functionally distinct lymphokine with the ability to act as a B cell growth factor.