We report a rapid method for screening transgenic mice by polymerase chain reaction (PCR) with small amounts of blood. Ten to 50 microliters of the uncoagulated blood were collected from the transgenic mice carrying the human A gamma/beta-globin gene (8.9 kb, 13 and 53 copies) by puncturing the leg vein or cutting the tail end. Blood was mixed well with 200 microliter of the white blood cell separating-solution and was stood for 40 minutes. DNA was extracted from the blood cells in the supernatant and was used for PCR. When 10-20 microliters of blood were applied to the cell separating-solution and the blood cells that washed out from 10-20 microliters of the supernatant were used for PCR, the specific DNA band that amplified from the transgene was always detected by the electrophoresis with an agarose minigel.