Genistein, an isoflavone, specifically inhibits protein tyrosine kinases (PTK). In low concentrations, genistein enhances natural killer (NK) cell lytic activity of peripheral blood leukocytes (PBL) in a dose dependent manner. The following study was designed to investigate if the enhancement of lytic activity was associated with the modulation of PTK's by genistein. A flow cytometric assay was developed to monitor phosphorylation of intracellular tyrosine residues in a pure population of NK cells (NK 3.3) in parallel with a chromium release assay for lytic activity. The flow cytometric assay used an anti- phosphotyrosine-FITC conjugated antibody in conjunction with the phosphatase inhibitor, pervanadate. Genistein at concentrations 0-10 uM was found to inhibit baseline phosphorylation and phosphorylation in the presence of pervanadate. However, while tyrosine phosphorylation was inhibited, the lytic ability of NK 3.3 cells against K562 cells was not affected. Since tyrosine phosphorylation provides an early and requisite signal for NK cell lytic activity, these results suggest that another pathway is utilized during activation of lytic activity. Thus, genistein does not appear to enhance lytic activity of NK 3.3 cells by modulation of their PTK'S.