Action of cytogenin on macrophages and T cells was investigated. Phagocytosis of yeast and production of PMA-elicited superoxide anion by macrophages taken from mice given cytogenin po were augmented. Cytogenin enhanced productions of IL-1 alpha by macrophages and IFN gamma and GM-CSF by spleen cells although it did not enhanced production of TNF alpha by macrophages and IL-6 by macrophages and spleen cells. Macrophages stimulated with cytogenin caused to stimulate proliferation of purified T cells in Intercell cultures in which each cell population was cultured without contact. Results suggest that cytogenin primarily activates macrophages to produce monokines such as IL-1 alpha and it causes to stimulate proliferation and differentiation of T cells resulting in production of lymphokines such as IFN gamma and GM-CSF.