Incubation of nitrocellulose filters containing proteins in solutions of organic alcohols (ethanol, methanol, isopropanol) enabled to increase the immunoblotting sensitivity after denaturating electrophoresis in presence of SDS. Maximal elevation of the label sensitivity (4-6-fold) was observed after incubation of these filters in 30% isopropanol within 2 hrs. The effect of sensitivity elevation appears to be caused by the antigen renaturation due to SDS washing off. The modified procedure of immunoblotting allowed to detect phenylalanine hydroxylase antigen in human thrombocytes. The antigen had electrophoretic mobility similar to that of phenylalanine hydroxylase from liver tissue; its concentration constituted less than 0.1 microgram per 1 mg of protein in thrombocytes.