The accuracy of the radiation-inactivation technique for estimating molecular size was investigated with a range of proteins of known molecular mass. With the use of irradiation with a 16 MeV electron beam, inactivation was examined both in frozen samples at 77 K and in freeze-dried samples at room temperature. The effect of the presence of detergents and chloroplast membrane preparations was also measured. It was demonstrated that proteins added as internal standards, including malate dehydrogenase, glucose-6-phosphate dehydrogenase and cytochrome c, can provide an accurate calibration of molecular size. However, a disadvantage of the technique was that the target size of oligomeric enzymes could be that of either the monomers, dimers or higher oligomers. The detergent Triton X-100 increased the rate of inactivation of the proteins investigated.