Natural killer cell activity (NKa) and mitogen-induced cellular cytotoxicity (MICC) of peripheral blood mononuclear cells (PBMC) were studied in five patients with chronic LGL-proliferative disease (LGL-PD) of the CD3+, CD8+, CD57+ phenotype. Both assays were performed under the same experimental conditions except that cultures for MICC contained phytohemagglutinin (PHA) at varying concentrations. Cytotoxicity was assessed against K562 cell targets using the 18 hours 51-chromium release assay. We found that LGL-PD lymphocytes of the aforementioned phenotype express low NKa but high MICC. Furthermore, supernatants derived from patients' PMBC cultures stimulated with PHA, displayed cytolytic properties comparable to those of normal lymphocytes. The findings indicate that MICC may be mediated, at least partially, by humoral cytolytic molecules. We concluded that LGL-PD lymphocytes are unable to express natural cytotoxicity but they have not lost the cytolytic machinery necessary for the destruction of sensitive target cells.