To analyze whether or not methyl methacrylate is immunologically inert, peripheral blood mononuclear cells were cultured with finely pulverized methyl methacrylate. Phytohemagglutinin (PHA) lectin, purified protein derivate of tuberculin (PPD) antigen, and culture medium alone were used as positive and negative controls. Lymphocyte kinetics on culture Days 0, 1, 3, and 5 were studied. Major histocompatibility complex locus II antigen (MHC locus II antigen; Ia) and interleukin-2 receptor (IL-2R; Tac) expression were analyzed using the avidin-biotin-peroxidase complex (ABC) method and lymphocyte DNA synthesis using 3H-thymidine incorporation and beta-scintillation counting. On culture Days 1 and 3, lymphocytes and monocytes were seen under the light microscope to be attached to methyl methacrylate particles. However, the results disclosed no methyl methacrylate-induced DNA synthesis, although methyl methacrylate-induced MHC locus II antigen and IL-2R activation marker expression were recorded; notably, this expression was less pronounced than that seen in PHA or PPD antigen driven lymphocyte response. The results suggest that methyl methacrylate is essentially an immunologically inert implant material. However, it seems to induce inflammatory mononuclear cell migration and adhesions leading to slightly nonspecific lymphocyte reaction.