Twenty-two SLE sera were assessed for their stimulating properties for lymphocytes. After fixation of specific serum factors, lymphocyte cultures were performed and supernatants of 24, 48, 72 and 96 h tested on two polymorphonuclear functions: chemotaxis and cyanide insensitive O2 consumption; 20 out of 22 SLE sera supernatants led to enhanced chemotaxis. The factors involved were not destroyed by heating for 30 min at 56 degrees C. Fifteen of the SLE sera supernatants increased the O2 consumption by PN. In most cases this second mediator was destroyed by heating. The buffer, the normal sera, the monoclonal antibodies stimulated lymphocytes supernatants and the total lymphocyte lysate did not display such enhancing activities. So it clearly appears that SLE sera contain factors acting on lymphocyte membrane and induce them to secrete new lymphokines called PNAF (polymorphonuclear activating factors) which may play a role in pathogenesis of tissue lesions in SLE.