Some characteristics of lymphocyte 5'-nucleotidase are reviewed. The optimal conditions for the cytochemical localization of 5'-nucleotidase (AMPase) in the mouse lymphocyte have been established. Quantitative monitoring of the effects of fixation and the components of the cytochemical medium showed that cytochemistry can be performed under conditions that do not lead to loss of AMPase activity. The cytochemical reaction product was seen only on the surface of a proportion of splenic lymphocytes, regardless of the fixative used. The splenic cell population included a distinct population of lymphocytes with readily demonstrable AMPase activity and another population with no cytochemically demonstrable AMPase activity. The number of positive cells varied, but was usually between 20 and 30% when cells were fixed in glutaraldehyde. Lymphocytes purified from thymus were always negative for cytochemically demonstrable AMPase activity. Biochemically, it was shown that the AMPase activity of spleen lymphocytes is more than six times higher than that of thymus lymphocytes. 5'-Nucleotidase activity in normal and abnormal lymphocytes is discussed in the light of the latest findings. The possible function of 5'-nucleotidase in lymphocytes is discussed.