As a serial experiment of radiosterilization of medical products, we investigated the feasibility of using gamma ray to sterilize antibiotics, tetracycline HCl. Tetracycline HCl in aqueous solution and dried state irradiated with several dose levels was assayed for physico-chemical properties by checking UV absorption spectra, paper chromatogrph. IR spectra and NMR sepctra, and antibiotic activities by [means of agar plaque technique and tube $d_1$]ution method. In physico-chemical propertis and biological activities, the teracycline HCl in dried state remained radioresistant up to exposure of 10 Mrad, however, in aqueous solution, the dose of 300 Krad produced a significant inactivation. From the results of this study, radiosterilization of tetracycline HCl is more recommendable in the dried state than in aqueous solution.