The test of spontaneous rosettes with sheep erythrocytes was used to estimate the rosette forming lymphocytes (RFL) in the peripheral blood from normal subjects and from patients with various lymphocytoses. The test was performed in 65 patients with: chronic leukemia (CLL), lymphocytic lymphosarcoma, Waldenström's macroglobulinemia, infectious mononucleosis, pulmonary tuberculosis, hyperthyroidism and pertussis as well as in 25 normal controls. The percentage of RFL was constantly and significantly low in all the cases of CLL. However, in absolute values the RFL count was found within normal limits or increased. Trying to establish a correlation between the number of T lymphocytes and the clinical forms of disease (aggressive and non-aggressive) on the one hand, and on the other hand to find the effect of cytostatic therapy, on this lymphocytic population, we observed that in the aggressive form the absolute values of RFL were obviously increased as compared with the other form and that cytostatic therapy reduced the number of RFL. The cellular immune deficit in CLL is discussed as well as the clinical utility of the rosette test with sheep erythrocytes for the estimation of the immune status in patients with malignant or reactive lymphoproliferations.