The distribution of T-lymphocyte subsets of 18 patients with lymphocytic leukaemia tested with monoclonal antibodies as well as E-rosettes formations and EAC-rosettes formations were studied. The patients classified according to RAI (stages 0-II. and III-IV.) a proportional decrease of T-lymphocytes was observed only, whereas their absolute number increased. T-lymphocyte subsets also changed: the ratio of CD4 positive lymphocytes reduce, while the proportion of CD-8 positive lymphocytes increased. The ratio of the two cell groups was below the normal value (1.8 and 1.0, respectively). This value is lower in stages III-IV., and refers to a serious immune imbalance, the latter being responsible for acute infections. The four weeks medication with Leukeran and COP resulted in unchanged rates of pathological cells with a decrease in the number of lymphocytes. These phenomena primarily refer to clonal damage of the cell line, resulting in pathological T-helper and T-suppressor functions. Owing to the relatively long lifespan of the lymphocytes, only a prolonged cytostatic treatment can yield favorable results in therapy.