Ecto-5'nucleotidase (5'NT), adenosine deaminase (ADA), purine nucleoside phosphorylase (PNP) and deoxycytidine (CdR), deoxyguanosine (GdR), deoxyadenosine (AdR) and adenosine (AR) kinases have been measured in subpopulations of peripheral blood lymphocytes of eight healthy volunteers. The separation of B, T, T helper/inducer and T suppressor/cytotoxic cells was performed by means of density gradient centrifugation, E rosetting, passage through a nylon-wool column and antibody affinity chromatography utilising OKT8 and OKT4 monoclonal antibodies. ADA was significantly higher in T lymphocytes and 5'NT in B lymphocytes. Among T cell subpopulations, 5'NT activity was significantly higher (P less than 0 . 01) in T suppressor/cytotoxic (OKT8+) cells (32 . 9 units/10(6) cells than in T helper/inducer (OKT4+) cells (9 . 7 units/10(6) cells). Indeed, the 5'NT activity in T suppressor cells was similar to that in B cells. T helper cells tended, however, to have higher PNP and ADA activities than T suppressor cells but the differences were not statistically significant. No major differences were noted in kinase activities between any of the lymphocyte subpopulations.