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Cellular immune responsiveness, as measured by lymphocyte transformation in one-way mixed leucocyte cultures (MLC) and in phytohaemagglutinin (PHA) stimulated cultures was evaluated in forty patients with systemic lupus erythematosus (SLE) and in seventy-four normal controls. The effect produced by sera from these subjects on in vitro lymphocyte reactivity was tested on autologous cells and on homologous responding cells from a constant panel of ten healthy volunteers. The reactivity of lymphocytes from SLE patients to PHA and to a battery of allogeneic cells was significantly lower than that of normal controls. Sera from some SLE patients inhibited the MLC reactions, while in other cases a distinct stimulatory effect was found. It is suggested that virus-induced modifications of normal histocompatibility antigens cause the appearance of blocking antibody that might bind to the surface of T lymphocytes, impairing their function. [ABSTRACT FROM AUTHOR]