When spleen cells of C57BL/6 mice or mast cell-deficient W/Wv mice were cultured, their histidine decarboxylase (HDC) activity increased with increases in the histamine concentration in the cells and the medium. Addition of concanavalin A (Con A) or Escherichia coli lipopolysaccharide (LPS) enhanced the increase. The removal of adherent cells reduced both the control HDC activity and the response to the mitogens. Purified T lymphocytes responded to Con A but not to LPS. Neither Con A nor LPS had any effect on B lymphocytes. Treatment of T cells with anti-Thy-1.2 and complement completely abrogated the induction of HDC. Histamine synthesis dependent on Con A by T cells was stimulated by the addition of conditioned medium from peritoneal adherent cells activated with LPS. The addition of recombinant interleukin-1 (rIL-1) or peritoneal adherent cells fixed with paraformaldehyde significantly enhanced HDC induction dependent on Con A in T cells. These results suggest that histamine is synthesized by T lymphocytes through HDC and that the reaction was enhanced by a soluble factor(s) released from macrophages.