Human erythrocyte (RBC) catechol-O-methyltransferase (COMT) is under genetic control. Experiments were performed to determine whether COMT in the human lymphocyte is regulated in parallel with RBC COMT. Supernatants of lymphocyte homogenates contained COMT activity. However, they also contained a potent COMT inhibitor, the effect of which could be negated by dilution. Lymphocyte COMT activity was maximal at a reaction pH of 7.7 and at a MgCI2 concentration of 0.67 raM. The apparent K m value for 3,4-dihydroxybenzoic acid, the catechol substrate for the reaction, was 1.2 ร 10 -5 g and that for S-adenosyl-L-methionine, the methyl donor, was 2.3 ร 10 -6 M. An average of 48.3 +_ 3.3% (mean +_ SEM) of the enzyme activity in crude lymphocyte homogenates from 3 subjects was removed by centrifugation at 100,000 g for 1 hr and was presumed to be membrane associated. The average COMT activity in lymphocytes isolated from blood of 23 randomly selected adult subjects was 14.0 +_ 1.2 units/lO e cells (mean +_ SEM) or 913 +_ 69 units/rag protein. There was a significant correlation of relative RBC with relative lymphocyte COMT activity in these 23 subjects. The correlation coefficient was 0.733 (P < 0.001) when lymphocyte enzyme activity was expressed per milligram of protein and 0.649 (P < 0.001) when lymphocyte activity was expressed per 106 cells. These results are compatible with the conclusion that the genetic polymorphism which regulates RBC COMT activity may also regulate the level of human lymphocyte COMT activity.