Intravenous Immunoglobulin (IVIG) at a concentration of 5 mg/ml, significantly inhibited mitogenic responses to phytohaemagglutinin (PHA), concanavalin A (conA) and pokeweed mitogen (PWM) by peripheral blood cells from healthy donors. No difference in inhibition by IVIG was seen when stimulating different T-lymphocyte cell subsets. Inhibition by IVIG was dose-dependent. An increased response was observed when IVIG was added more than 12 h after PHA compared to adding 1 h before [P = 0.05]. Intravenous immunoglobulin added to mixed lymphocyte cultures (MLC), reduced the median response by more than 60% (range 14-89%; P = 0.03) and almost completely abrogated the lymphocyte response to Staphylococcus aureus protein A (SPA), whose median inhibition was 94% (range 90-99%; P = 0.02). When comparing 12 different commercial IVIG preparations at a concentration of 2.5 mg/ml, the median inhibition of the PHA stimulation ranged from 4% to 35% and the MLC response from 0% to 66%. In the presence of IVIG the lymphocyte response to different herpes virus antigens was reduced by > 50%. No difference in inhibitory effect was seen when comparing IVIG and cytomegalovirus (CMV) hyper Ig, but CMV negative Ig resulted in lower inhibition [P = 0.05]. Three out of five IgG preparations (2.5 mg/ml) made from single donors inhibited PHA stimulation significantly more than commercial IVIG [P < 0.05]. Mean inhibition was 61% compared to 35%. Inhibition by pooled IgG from five donors was 56%. F(ab')2 fragments of IVIG inhibited the MLC response by more than 50% (range 34-75%), SPA stimulation by 97% (83-104%) and PHA stimulation by more than 30% (26-37%). One of two Fc preparations tested had an inhibitory effect, but the inhibition was less than that obtained with the F(ab')2 fragments [P = 0.04]. These results further strengthen the notion that IVIG exerts its immune modulatory effect by binding to leukocyte surface receptors. A clear inhibition was obtained with concentrations corresponding to the serum levels obtained when IVIG is given 250-500 mg/kg bodyweight. F(ab')2 fragments have the same inhibitory effect as intact IgG molecules but the role of Fc fragments still remains unclear. Differences in the immunosuppressive effect of various IVIG preparations may be associated with the method of preparation.