A case of T-cell chronic lymphocytic leukaemia (T-CLL) with an unusual mature membrane phenotype: E+, CD3+, CD4+, CD8-, M1+, Leu-15+, Fc gamma+, is described. The cells were large granular lymphocytes with slight immature features. Functionally these cells lacked helper, suppressor and NK activity but possessed normal levels of K activity. These findings demonstrate several features not previously described in T-CLL: the coexpression of the antigens detected by T4, M1 and Leu-15 the presence of Fc gamma receptors on CD4+ lymphocytes and the lack of NK activity in M1+, Fc gamma+ cells. This study broadens the known heterogeneity of T-CLL and suggests the existence of a hitherto unrecognized normal T-lymphocyte subset with the same functional and phenotypic characteristics as in the case described here.