In the prophylaxis of post-transfusion GVHD, it is recommended to irradiate blood components with an absorbed dose ranging from 15 Gy to 50 Gy. We applied a new chemical dosimeter to measure absorbed dose for blood irradiation. This system is composed of diphenyliodoniumchloride (DICI) as an oxidizer and crystal violet lactone as a pH indicator. Protons are released from DICI after irradiation. The maximum absorption wavelength of this indicator is 610 nm, and absorbance is linearly related with a given dose from 1 to 50 Gy. Coloration is stable for at least 2 weeks when stored in the dark at 4 degrees C. This system may be used as a chemical dosimeter for blood irradiation.