The phenotypic and functional features of purified large granular lymphocytes (LGL) from ten patients with LGL population expansions and cytopenias are described. The predominant LGL phenotypes were T3+, T8+, Leu-11+/-; however, in two patients, LGL expressed a T3-, Leu-11+ phenotype. Variable combinations of other LGL markers (OKM1, Leu-7), and HLA-DR were detected in individual cases. In nine of ten cases, freshly isolated LGL did not exert cytolytic activity for K562 target cells, but purified LGL cultured in the presence of recombinant interleukin 2 (rIL2) acquired potent cytotoxic activity in all cases tested. LGL did not proliferate in response to phytohemagglutinin (PHA). However, LGL released variable amounts of IL2 and gamma- interferon (gamma-IFN) after PHA stimulation. In some cases, stimulation of fresh LGL with recombinant IL2 induced production of gamma-IFN. No correlation was found between the functional capabilities and the original phenotype of the expanded LGL populations.