Concanavalin A induced polyclonal antibody synthesis in normal spleen cells in vitro. Optimal responses were obtained by Con A concentrations lower than those optimal for induction of DNA synthesis. T cells, but not macrophages, were necessary for the effect. Spleen cells from nude mice were not activated, whereas cells from the LPS non‐responder stain C3H/HeJ were activated to polyclonal antibody synthesis by Con A. Supernatants from Con A activated spleen cells could by themselves induce polyclonal antibody synthesis in untreated spleen cell cultures, even when Con A had been removed by absorption with Sephadex G‐50 and when alpha‐methyl‐mannoside was present in the secondary cultures. T cells produced the active Supernatants, which were competent to induce polyclonal antibody synthesis, but not DNA synthesis, in both H‐2‐incompatible and compatible strains. When the Supernatants were absorbed with erythrocyte antigens, they specifically induced an enhanced response, in secondary cultures, to the antigen used for absorption. Possible mechanisms of this specific effect are discussed. Copyright © 1979, Wiley Blackwell. All rights reserved
