The reduction in all activities at 100 μg/ml was not due to the toxicity of the extract , because the viability of PBMC , as determined by tryphan blue staining, was not different from the corresponding control cultures . The cell viability of each donor was 95% or more in the presence or the absence of the extracts (data not shown). It was possible that the D. scandens extract at a concentration of 100 μg/ml or higher could induce secretion of cytokines and/or other soluble factors that have inhibitory effects on lymphocyte proliferation and NK cell activity.