Supernatants from clones of human T lymphocytes that were responding to a purified Mycobacterium tuberculosis antigen were able to activate macrophages and macrophage-like myeloma cells (U937) to release increased amounts of the microbicidal agent hydrogen peroxide. The activity was not neutralized by monoclonal antibody against interferon-gamma (IFN-gamma), was greater than could be accounted for by the IFN-gamma activity in the supernatants, and was separated from IFN-gamma by high performance liquid chromatography. It is evident that IFN-gamma is not the only macrophage activator released by T lymphocytes responding to microbial antigen, and may not even be the main one to enhance antimicrobial activity in infections such as tuberculosis.