TOTP is a prevalent industrial and environmental contaminant which has been shown to be a neurotoxic agent. This study was undertaken to investigate the effects of this compound on the immune system. Several techniques were employed to test the humoral as well as cellular effects. Male Swiss Webster mice were administered by gastric gavage with three doses of TOTP (5, 50, and 500 mg/kg) and one dose TMTP (50 mg/kg) in corn oil once a week for 13 weeks. Control animals were given corn oil alone. Lymphocyte transformation was determined on cultures of splenic cells obtained from animals sacrificed at 1, 4, 8, and 13 weeks. Relative Proliferation Index (RPI) and Stimulation Index (SI) was calculated for phytohemagglutinin (PHA), pokeweed (PWM), and lipopolysaccharide (LPS) mitogen treated cultures. Proliferation was measured by uptake of radioisotope labeled thymidine. Plaque formation as well as delayed hypersensitivity was evaluated at all dose levels after 4 weeks of treatment. Humoral effects were determined utilizing quantitative (Rocket) immunoelectropheresis. Serum was compared with control serum for IgA, IgG, and IgM fractions. TOTP was found to cause slight suppression in proliferation of splenic lymphocytes at 8 and 13 weeks although this suppression was non-specific and not dose related. No effects were seen on body and organ weights. Plaque forming cells and delayed hypersensitivity were not affected either by dose or time of treatment. Immunoglobulin fractions tested were similarly not affected by time or dosage. These findings suggest that neither TOTP nor TMTP exhibit immunotoxic effects at levels used.