Levamisole [(S)-(-)-2,3,5,6-tetrahydro-6-phenyl-imidazo-(2,1-b) thiazole], an immunomodulating drug and veterinary antihelminthic, is converted by tissues to a sulfhydryl derivative. The drug and its metabolite have mediating effects on lipid peroxidation in microsomal preparations. Because levamisole, as an inhibitor of lipid peroxidation, is a radioprotectant drug, it was of interest to study the response of the drug itself to ionizing radiation. Experiments were directed toward an examination of the effects of gamma radiation on aqueous solutions of levamisole. Chromatographic analysis (TLC) revealed two distinct groups of radiation products. Further separation and analysis of these groups by gas chroma- tography-mass spectrometry (GC-MS) demonstrated that each group of radiation products consists of several components, indicating that the gamma irradiation of non-deaerated solutions of levamisole gives rise to varying amounts of a multiproduct mixture, no constituent of which corresponds to the natural metabolite. Dose effect curves for the levamisole irradiation indicate that the drug is markedly resistant to molecular alteration under the experimental radiation conditions.