DAP-IV activity (Gly-Pro-MCA hydrolysis, pH 7.8) was found in lysates of peripheral blood lymphocytes of patients with T- and B-cell forms of malignant lymphoproliferative diseases. The highest DAP-IV activity was seen in the cells of patients with a rare variant of T-cell lymphocytic leukemia (T-CLL); these cells expressed simultaneously the antigens of T helpers and T suppressors (Th and Ts) (OKT4+ and OKT8+). The DAP-IV activity about ten times less was found in the pathological cells with a phenotype of mature Th (Sezary disease), as well as in the cells expressing antigens of both Ts and natural killers (a rare variant of T-CLL). The same activity was also found in Ts (T gamma-lymphocytosis). The data obtained show that the differences in DAP-IV expression are connected with the differentiation step rather than with the belonging to a particular subpopulation of T-cells. DAP-IV activity, which was somewhat lower than that of T-cells, was found in B-lymphocytes of patients with B-CLL, hair-cellular leukemia, and non-Hodgkin's lymphoma. No correlation of DAP-IV activity with the level of E-cellular differentiation was observed.