We examined the ability of macrophages and B cells to function as antigen-presenting cells (APCs) for murine T(H)1 and T(H)2 cloned T helper cell lines. Antigen presented by concanavalin A-elicited peritoneal macrophages or resting splenic B cells stimulated antigen-dependent proliferation of both T helper subsets. Paraformaldehyde fixation of the APCs following different conditions of activation indicated differential requirements for costimulatory signals by T(H)1 and T(H)2 cells. T(H)2 proliferative responses were strictly dependent on APC expression of IL-1. T(H)1 proliferation was dependent on APC expression of a non-IL-1 costimulatory signal present on freshly isolated macrophages and on splenic B cells activated with anti-immunoglobulin plus interferon γ.