Analysis of data on 9 cases with active cytomegalovirus infection in patients with kidney grafts showed a positive association of serum leucine aminopeptidase activity concentration with the appearance of plasmacytoid lymphocytes in blood. Additional studies indicate that (1) like the liver, the lymphocytes contain leucine aminopeptidase in relatively large quantities and that (2) this enzyme is increased about 3-fold in plasmacytoid lymphocytes when compared with the activity in normal lymphocytes. In contrast, the ‘hepatic’enzyme alanine aminotransferase is practically absent in both lymphocytes and plasmacytoid cells. Therefore, the difference in serum between the relative increases of leucine aminopeptidase and alanine aminotransferase may be attributed to proliferating plasmacytoid lymphocytes. Earlier observations on a large number of cases of acute viral hepatitis A or B lend credence to this assumption. However, in this disease, the serum enzyme changes reflect the much greater involvement of the liver and the relatively slight, but significant, proliferation of plasmacytoid lymphocytes. Our hypothesis is confirmed by the recent observation of 3 cases of acute EBV infection (infectious mononucleosis) in otherwise healthy individuals showing greatly elevated leucine aminopeptidase in contrast to normal or slightly raised alanine aminotransferase in serum.