5′-Nucleotidase (5′-N) is known as a cellular ectoenzyme of wide tissue distribution. The varying expression of the enzyme in leukocytes is thought to be linked to cell differentiation. We, therefore, studied 5′-N activities of bone marrow or peripheral blood mononuclear cells in subgroups of leukemias and lymphomas as defined by morphology and immunological marker expression. In acute leukemias, elevated or very high enzyme activities were correlated with the expression of the common ALL antigen (cALLA). A similar correlation with cALLA was also observed for lymphoid blast crisis in CML. Therefore, 5′-N may be used as an additional diagnostic marker to discriminate cALLA positive leukemic cells from those not expressing cALLA. The 5′-N of leukemia cells of all patients tested gave complete crossreactivity with an antiserum directed against the normal human placental enzyme. Serum-5′-N was not well correlated with cellular enzyme activity in the same patient and therefore appears less suitable as diagnostic marker enzyme. However, elevated serum-5′-N was frequently observed in common ALL. Low enzyme activities in peripheral mononuclear cells were characteristic for some forms of lymphoma like CLL of B-type and hairy cell leukemia. In other lymphoma types, single patients with elevated or even extremely high 5′-N were found, for example in T-CLL, B-PLL and cc-lymphoma. With the limited data available it is too early to decide whether 5′-N could also become an additional useful biochemical marker for the classification of lymphoma subgroups. Though the investigations clearly indicate that 5′-N is not a specific marker for a leukemia or lymphoma subtype, the biochemically measurable variance of its expression appears to be a good indicator for the maturation degree of lymphatic cells. [ABSTRACT FROM AUTHOR]