Peripheral blood lymphocytes (PBL) were separated into two subclasses by differential adherence to wheat germ agglutinin (WGA). WGA-adherent PBL differed structurally (different WGA-binding properties) and functionally from WGA-non-adherent cells. As judged by [3H]-thymidine incorporation, WGA-adherent PBL responded less well to L-phytohaemagglutinin (L-PHA) than non-adherent cells. This difference was not due to different L-PHA dose requirements or different response kinetics. WGA-adherent and non-adherent PBL bound identical amounts of 125I-L-PHA and contained comparable percentages of T cells, B cells, and monocytes. Addition of mitomycin-C-pre-treated WGA-adherent cells to non-adherent cells caused suppression of the L-PHA response. Maximal suppression occurred at a ratio of 1 adherent:2 non-adherent cells and on days 5-7 of culture. Under these conditions the adherent cells themselves did not proliferate indicating that active proliferation was not required for inhibition. Suppression was selective for L-PHA as it did not occur in cultures stimulated with concanavalin A, pokeweed mitogen, Lens culinaris lectin, or in the mixed leucocyte reaction. Cell fractionation techniques indicated that plastic adherent cells (presumably monocytes) in the WGA-adherent subclass were critical for mediation of suppression; These data provide evidence for a specific human suppressor cell of the in vitro response to L-PHA.