The mechanism by which B and T lymphocytes interact synergistically in the proliferative response to 2-mercaptoethanol (2-ME) as a mitogen was investigated in cultures of C3H/St spleen cells. The interaction between these cells required physical contact between the collaborating cell types, and was not mediated by the release of a soluble factor into the culture supernate. Sonicates of spleen cells which had been activated with optimal concentrations of 2-ME for 24 h and then washed extensively, stimulated the uptake of tritiated thymidine and morphological blast transformation of fresh, unstimulated cells. This activity was found to reside within the soluble fraction of the activated cells, and to activate cells optimally at a ratio of 1 naive cell: 1 activated cell-equivalent. This reciprocally-acting lymphocyte proliferation helper (RALPH) activity was produced by B cells as well as by T cells, with a kinetic peak at 48 h of culture. RALPH activity was produced by viable but not by nonviable cells incubated with 2-ME, and was nondialyzable. It could not be induced by the B-cell mitogens lipopolysaccharide, polyinosinic-polycytidilic acid, or purified protein derivative of tuberculin, or by the T-cell mitogen concanavalin A. RALPH isolated from T cells activated B cells exclusively, while that from B cells acted predominantly upon T cells, possibly with a nonspecific effect on B cells. A model for the cellular interactions involved in the amplification of the proliferative response to 2-ME is described.

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