The effect of alcoholic patient sera on in vitro lymphocyte transformation was studied using mitogen-induced uptake of 3H-thymidine to measure blastogenesis. With pokeweed mitogen as the stimulus, transformation of normal lymphocytes in sera of alcoholics with either normal or fatty livers was not significantly different from that obtained in pooled human serum (PHS). However, in sera of patients with either alcoholic hepatitis or inactive cirrhosis mean transformation was significantly reduced (P <0.001, <0.02 respectively). With phytohaemagglutinin-P or concanavalin A as mitogens, suppression of transformation was not as marked but followed the same pattern. A significant negative correlation was observed between the magnitude of transformation and serum bilirubin and aspartate aminotransferase levels. An intra-patient comparison of the effects on transformation of normal lymphocytes by simultaneously collected peripheral and portal venous sera, and of peripheral sera obtained before and after portasystemic shunt surgery, indicated that the factor(s) responsible did not originate in the splanchnic circulation nor did it accumulate in the serum because of failed hepatic clearance. By performing transformation experiments in the presence of inhibitory patient sera diluted with PHS it was possible to show that these sera caused true inhibition of transformation rather than suppression due to failure to sustain cell culture because of nutritional deficiencies. Inhibitory sera did not contain high levels of the enzyme thymidine phosphorylase and did not significantly inhibit binding of 125I-labelled mitogens to the lymphocyte surface. These findings indicate that the inhibitory effect of sera from alcoholics is of potential in vivo importance, that the effect increases with the degree of hepatocyte damage, and that it is unrelated to the nonhepatic metabolic affects of chronic alcoholism.