The development is reported of a reproducible colorimetric irradiation dosimeter that is easy to prepare as well as to interpret. Optimal chloroform, dithizone, and paraffin concentrations to produce a distinctive color change at > 1500 cGy (optimized for 3000 cGy) were determined by combining various concentrations of each component at 65 degrees C. The melted dosimetric material was poured into molds, allowed to solidify, and irradiated with doses ranging from 0 to 3000 cGy. Color change was evaluated visually and spectrophotometrically to determine reproducibility. Twenty-percent chloroform (wt/wt) and a dithizone concentration of 5.0 x 10(-5) M combined in paraffin (TP-2) produced optimal color change from pink to green after > 1500 cGy. The change was reproducible, and 50 individuals were able to distinguish between irradiated and nonirradiated dosimeters. Additionally, five of five of these individuals correctly ranked five dosimeters in order of increasing irradiation from 0 to 3000 cGy, in increments of 750 cGy. This dosimeter is easy to make and easy to read and may allow blood banks to show unit-by-unit quality assurance for irradiated blood components and quality control of the irradiator itself.