Fas receptor activation induces tyrosine phosphorylation of several cellular proteins in human eosinophils. Freshly isolated eosinophils were stimulated with anti-Fas MoAb for the indicated times. The cell lysates were immunoprecipitated with 4G10 MoAb, and the ptyr immunoprecipitates were analyzed by immunoblotting with a MoAb to ptyr, RC20. Increased tyrosine phosphorylation of 48-, 60-, 72-, 82-, 88-, and 98-kD proteins (arrows) was observed. The position of molecular size standards (left). Data are representative of three independent experiments.