Sterility and antibacterial activity of several antibiotics (including some penicillins and their salts, gramicidin and neomycin) subjected to sterilization by irradiation has been studied. The compounds in solid phase have been exposed to gamma irradiation in air atmosphere at room temperature, with a dose of 25 kGy, and afterwards they have been subjected to tests recommended by FP V (volume I, 1990) checking their sterility and activity. The results have shown that the majority of initial compounds have been to a slight degree contaminated by bacteria from the genera Bacillus and Micrococcus, the number of bacteria did not exceed 10(2) CFU, and fungi up to 10 CFU in 1 g of the compound. All compounds subjected to sterilization with a dose of 25 kGy were sterile and preserved the activity required by FP V. The decrease in activity observed for some compounds was always within the limits of FP specification. The results have proved that the penicillins analysed, gramicidin and neomycin can be sterilized by irradiation with a dose of 25 kGy, without any detrimental effect on their properties and antibacterial activity.