Eosinophils are emerging as an increasingly important cell in the immunoregulatory network of normal and pathological processes. No studies has yet described optimized experimental strategies to transfect DNA into human eosinophils. Using a frequently employed in vitro model of human eosinophil, the EoL-1 cells, we now described the optimal transfection of DNA into these cells by electroporation. Our results indicate that electroporation can efficiently and reproducibly transfect DNA into EoL-1 cells. Optimal electroporation conditions consist of the use of 1 X RPMI medium 1640 with 10% FBS, voltage setting at 275 V, 1150 microF capacitance, 40 mg of DNA and 4.0 X 10(7) cells/ml per electroporation in a total volume of 0.5 ml in 0.4 cm gap cuvettes. These conditions may be a useful protocol for transfecting eosinophil cell lines.