Production of interferon (IFN)-gamma was investigated in human peripheral lymphocytes stimulated with Mycoplasma pneumoniae. Lymphocytes obtained from non-immune individuals produced no IFN. IFN-gamma was produced by T cells obtained from immune individuals, and the helper/inducer T cells produced two- to sixfold higher titer of IFN-gamma than the suppressor/cytotoxic T cells. The addition of macrophages in T cell cultures suppressed the production of IFN-gamma; this differs from the previous result wherein the addition of macrophages enhanced the production of IFN-gamma, when stimulated with mumps virus or measles virus.