Human peripheral blood lymphocytes from healthy individuals were examined to detect the presence of membrane-bound Ig and surface receptors for fixed IgG and fixed C3. These three features are thought to be characteristic of B lymphocytes and were measured by the mixed antiglobulin rosetting reaction, the antibody (Fc) rosetting reaction and complement (C3) rosetting reactions, respectively. As a reaction believed to be characteristic of T lymphocytes, the capacity to rosette with non-sensitized SRBC was also evaluated. Mean percentages of the total number of lymphocytes from 20 healthy donors ± 1 SD were: sheep red blood cell rosetting reaction 55.7 ± 9.6; mixed antiglobulin rosetting reaction 34.3 ± 9.6; Fc rosetting reaction 26.3 ± 9.9, and C3 rosetting reaction 14.7 ± 5.2. Having established a range of normal values, eleven patients with chronic lymphatic leukaemia (CLL) were examined. For all the reactions believed characteristic of B lymphocytes the mean percentage of lymphocytes rosetting was increased; mixed antiglobulin rosettes 85.8; Fc rosettes 60.4, and C3 rosettes 56.4. There was consequent reduction in the mean percentage of lymphocytes rosetting with SRBC, i.e. 11.8%. This confirms that, in the cases studied, CLL is a disorder of B lymphocytes. In normal persons and in CLL patients it was found that Fc receptors, C3 receptors and Ig determinants appeared to express themselves independently on B lymphocytes.