Three subpopulations of human T lymphocytes (TG, TM, and Tnon-G/non-M cells) were evaluated by means of six different cytochemical reactions. Elevated staining activity was demonstrated in all the populations when stained for acid phosphatase (AcP), beta-glucuronidase (BG) and N-acetyl beta-glucosaminidase (NABG). While the majority of TM cells displayed an intense acid alpha-naphthyl acetate esterase (ANAE) positivity, only a smaller proportion of the Tnon-G/non-M exhibited a positive reaction and a much lower activity was found in the TG group. TM cells were also alpha-naphthyl acetate esterase (NAE)-and alpha-naphthyl butyrate esterase (NBE)-positive, showing a dot-like pattern. These reactions were present in a lower proportion in the Tnon-G/non-M population and nearly completely absent in the TG cells. The present study identifies different cytochemical phenotypes for the different T-lymphocyte subsets and the analysis of these phenotypes may indicate that the TM is the most cytochemically differentiated subpopulation.