The specific activity of S-adenosyl-L-methionine: protein-lysine N-methyltransferase (Protein methylase III; EC 220.127.116.11) was more than 2-fold higher in T-lymphocytes than in B-lymphocytes isolated from human spleen. Further analysis revealed that T8 cells contained more than twice the amount of the enzyme activity found in T4 cells. Stimulation of T-cells with pokeweed mitogen (PWM) did not increase the enzyme activity. In contrast, however, while the specific activity of S-adenosyl-L-methionine: protein-arginine N-methyltransferase (Protein methylase I; EC 18.104.22.168) was about equal between the T- and B-lymphocytes, this enzyme activity was enhanced more than two-fold by administration of either PWM or Staphylococcus aureus. The endogenous protein of human T-lymphocytes enzymatically (methyl-3H)-labeled by protein methylase III was found not to be histone, the molecular size being less than 14,000 Da.