Natural killer (NK) activity of peripheral blood mononuclear cells (PBMC) from 28 patients with systemic lupus erythematosus (SLE) was measured using enzyme-release assay. The SLE patients had significantly decreased NK activity in comparing with normal controls. The levels of NK activity correlated with disease activity. Pre-incubation of PBMC separately with Astragalus membranaceus and Tripterygium hypoglaucum or with their mixture considerably stimulated NK cytotoxicity both in SLE patients and healthy donors. The extent of enhancement was dose-dependent and relevant to pre-incubation periods. The release of a soluble natural killer cytotoxic factor (NKCF) by peripheral blood mononuclear cells was tested by cytotoxicity assay induced in K562 cells. Natural killer cytotoxic factor release was significantly lower in SLE patients than in controls. The levels of natural killer cytotoxic factor were correlated well with NK activities, but correlated negatively with clinical activity. Pre-incubated supernatants from peripheral blood mononuclear cells with above-mentioned agents caused much higher percentage of lysis on K562 targets than that of without pre-incubation.