During the years 1944–1945, while working on the Manhattan Project at the University of California Radiation Laboratory at Berkeley, the author had access to classified reports which included, among other things, the effect of high-energy radiation on components of nuclear piles, such as electrical insulation. However, in these reports no attempts to determine the actual molecular mechanisms of the radiation effects were described. Consequently, it was rather obvious that this was a wide open area for research, so when the author returned to his normal duties with the Faculty of Chemistry at Northwestern University, the decision was made to begin such a study. With the support of the Visking Corp. of Chicago (now a part of Union Carbide), such a research was begun. The Visking Corp. supplied financial support as well as films of low-density polyethylene (PE) for our study. With the aid of the graduate student Donald R. Rose these films were irradiated with a mixture of gamma and neutron rays in the heavy water pile of the Argonne National Laboratory. After the irradiation, stress-strain curves on strips of the film were recorded using a Scott Inclined Plane tensile strength machine in the laboratories of the Visking Corp. The stress-strain curves obtained on samples before irradiation are illustrated in Fig. 1 while after irradiation in air the curves of Fig. 2 were obtained. The latter show a distinct effect of the irradiation, but at that time in 1948 we were not sure whether or not the diminution of the cold drawing ability of the PE was due to C-O-C cross-links or to true covalent C-C bond cross-links. Consequently, an irradiation was performed in vacuum with the even more striking results illustrated in Fig. 3. By comparing Figs. 1 and 3 it can be seen that the cold drawing ability of the PE was practically completely eliminated by the irradiation. These results were interpreted in terms of the formation of C-C covalent bond cross-links between the long PE chains. It is really remarkable that a reaction such ascould occur in the solid PE at room temperature. The discovery of the radiation cross-linking of PE was described at the Portland, Oregon, meeting of the American Chemical Society in September 1948 [1]. Other discoveries made and later presented at a Symposium of the Technical Command, Army Chemical Center, Maryland [2] were (1) radiation oxidation of the films, (2) formamation of trans-vinylene unsaturation, (3) evolution of hydrogen, etc. The expression “radiation damage” is heard, but in the case of PE we should speak of “radiation improvement” because the physical properties of PE, such as greater stability at higher temperatures, and increased tensile strength at low doses, are significantly improved. Furthermore, the radiation improvement of PE occurs at room temperature without having to melt the sample or to change its shape in any way. This is of considerable industrial importance.