The cytotoxic monoclonal antibody (Mab) LIL 13 reacts with a widely distributed antigen that is expressed on 80-95% of swine peripheral blood mononuclear cells (PBMC) and a variety of lymphoid cell types. Using indirect immunofluorescence (IIF) the positive cells (75-100%) were divided between the bright, intermediate and dull populations. The remaining negative cell population contained B-cells, T-cells and probably null cells. Mab LIL 13 did not react with swine major histocompatibility complex (MHC) class I antigens (SLA) and did not inhibit E-rosette formation. Reactivity of LIL 13 with leukocyte function antigen 1 (LFA-1) was excluded by competitive IIF and cytotoxicity tests with cross-reacting anti-human CD 18 or anti-swine LFA-1-specific antibodies. Mab LIL 13 and complement treatment severely reduced mitogen-induced proliferative response to phytohaemagglutinin (PHA), concanavalin A (Con A), pokeweed mitogen (PWM) and lipopolysaccharide (LPS) (90-100%). In the absence of complement, LIL 13 partially reduced proliferation of cells by interfering with the capability of mitogens to bind to the corresponding surface receptor (LIL 13 followed by mitogens), and partially inhibited mitogenic proliferative response following post-treatment (mitogens followed by LIL 13). Biochemical analysis of the antigen using sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) revealed bands of approximately 180-190 kDa and 46-50 kDa under reducing conditions and 200 kDa and 46-50 kDa under nonreducing conditions.