Different methods for determination of T-lymphocytes in human peripheral blood were compared: rosetting with sheep erythrocytes (SRBC), AET-treated SRBC, immunofluorescence using a monoclonal antibody against T cells (BL-T2), complement dependent cytolysis with polyclonal antisera against thymocytes, cytochemical demonstration of unspecific acid alpha-naphthyl-acetate esterase (ANAE), and electrophoretic mobility using a cell electrophoresis system (PARMOQUANT 2). Depending on the method, mean values between 70 and 79% T cells among separated mononuclear cells (MNC) were found. All paired observations were subjected to statistical analysis using rank correlation and U-test. From this analysis it is concluded that rosetting with SRBC, immunofluorescence using the monoclonal T-cell antibody and cytochemical reactivity for ANAE are favored methods for determining the T cell content of human MNC. However, the monoclonal antibody BL-T2 and the ANAE are not generally applicable because both markers were also found on malignant B-lymphocytes (B-CLL).