Generation of effector cells, the expression of cell mediated lympholysis (CML) and the reactivity of mononuclear cells in solid cultures, were explored in patients with Systemic Lupus Erythematosus (SLE). While proliferative responses to alloantigens were comparable to the controls, a significant decrease in CML capacity was found in SLE T lymphocytes; further, T cells stimulated with a T cell mitogen in solid cultures showed a diminished proliferative response to soluble factors. Functional cell interaction defects rather than intrinsic T cell abnormalities may be operating in SLE.