To further investigate the mechanisms accounting for defective natural killer (NK) activity observed in the majority of patients with lymphoproliferative disease of granular lymphocytes (LDGL), we have studied the generation of natural killer cytotoxic factor (NKCF) from peripheral blood lymphocytes recovered from twelve LDGL patients. On the basis of their cytotoxic activity against the K-562 target cells, cases under study were separated in two groups. Cells from five patients, referred to as NK+, were found to exhibit high levels of NK-cell mediated cytotoxicity; cells from seven patients, referred to as NK-, despite their ability to bind to the K-562 targets, displayed a defective NK function. The coculture of cells from NK+ patients with NK sensitive K-562 cells triggered a significantly higher production of NKCF with respect to NK- patients (p less than 0.001 at 1:2 dilution). Using phytohemagglutinin (PHA) as the NKCF stimulator, differences between the groups were not significant, due to a recovery of NKCF generation in NK- patients. Furthermore, a consistent lectin (PHA) dependent cellular cytotoxicity (LDCC) was exhibited by both groups of patients. The addition of gamma-interferon (IFN-gamma) or interleukin-2 (IL-2) during NKCF generation did not significantly modify the production of this factor. Our data point out that (a) NKCF is involved in the lytic activity of LDGL patients' cells, (b) a constitutional impairment in the generation of NKCF is not present in NK- patients, since recovery of lytic function occurs after PHA stimulation, and (c) IL-2 and IFN-gamma do not play a relevant role in triggering NKCF production when added during the generation of this factor. These studies help to clarify the factors involved in the cytotoxic machinery of LDGL patients' cells, suggesting that the defect of the cytotoxic function observed in some LDGL patients could lie at the level of target recognition.