The purpose of this study was to determine the effects of manual metal arc stainless steel welding fume particulates (MMA-SS) on the immune system. MMA-SS (Lincoln Electric Co., Cleveland, OH) were determined to consist of Fe and Ni in the insoluble portion, with Cr and Mn in the soluble portion. Total, soluble, and insoluble MMA-SS particulate were directly exposed to splenocytes derived from female B6C3F1 mice. Viability studies identified the EC50 value to be 6.4 ug/ml; MMA-SS concentrations of 5, 2.5 and 1.25 ug/ml were used for experiments. Significant decreases in the total IgM (antibody response to sRBC)/106 viable cells (87%, 67%) and total IgM/well (73%, 58%) was observed in the splenocytes exposed to 5 ug/ml of total and soluble MMA-SS welding fumes, respectively. Female B6C3F1 mice were administered MMA-SS by pharyngeal aspiration every 5 days over a 20 day period at concentrations of 10 mg/kg and 20 mg/kg with no observed changes in body weight and a 32% (p < 0.05) increase in lung weight. A dose dependent decrease in the number of AFC/106 cells and AFC/spleen in response to i.v. immunization with sRBC were observed after pretreatment with MMA-SS. The specific activity (IgM AFC/106 spleen cells) was decreased 18 and 33% following exposure of mice to 10 or 20 mg/kg respectively. A decrease in AFC/lung associated lymph nodes (LALN) was also observed in response to i.t. immunization with sRBC after exposure to MMA-SS. There was a 75% (p < 0.01) and 42% (p < 0.05) decrease in AFC /LALN after exposure to 20 mg/kg and 10 mg/kg, respectively. The immunosuppressive effects seen may be caused by the direct release of soluble MMA-SS metals or indirect actions mediated via the hypothalamic pituitary axis or due to the local inflammatory process.