In 20 patients with lymphoproliferative disease of granular lymphocytes (LDGL) the cytotoxic in vitro function of peripheral blood mononuclear cells was studied against both NK sensitive (K-562) and NK resistant target cells (Daudi). Cytotoxicity was analysed at resting conditions and after in vitro pre-incubation with recombinant alpha 2-interferon, gamma-interferon and interleukin 2. At resting conditions, a heterogeneous cytotoxic picture was observed against the K-562 targets, with 11 of the 20 patients showing a normal or increased NK activity, and nine an impaired function. Against the Daudi targets, unstimulated cells from all patients displayed a normal cytotoxic activity. Following in vitro stimulation, an increased lytic function against K-562 cells was demonstrated in six of the nine patients with low basal values, in three after boosting with all lymphokines tested and in three with interleukin 2. In three of these six patients incubation with interleukin 2 also produced an increased cytotoxic function against Daudi target cells; this phenomenon may be associated to the lymphokine-activated killer (LAK) system. The importance of investigating the cytotoxic in vitro function in LDGL patients to further characterize the biological features of GL and its relevance to better define the nature of this disease are discussed.