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The photodegradation of ceftriaxone (CFT) in aqueous solution induced by UVB irradiation was studied in the presence and in the absence of oxygen. These selected conditions simulate some natural situations of contaminated water, where dissolved oxygen and the fraction of UV radiation that penetrates the terrestrial atmosphere could participate in the photodegradative process. CFT undergoes unimolecular photodegradation when irradiated with UVB light, and in an oxygen atmosphere the photodegradation process increases due to the action of reactive oxygen species (ROS), which are generated by photosensitization processes that involve the CFT triplet excited state (3CFT*). We report here for the first time the transient absorption spectrum, the lifetime and the quantum yield of 3CFT*. In the photodegradative mechanisms, hydroxyl radicals, superoxide radical anions and hydrogen peroxide are generated and participate in the CFT photooxidation. Bioassays on the Vero cell line demonstrated that the CFT photoproducts generated in the presence and in the absence of oxygen are not cytotoxic. A decrease in the bactericidal capacity of CFT after UVB exposure against S. aureus strains was also observed. The photoproducts obtained under photooxidation mechanisms mediated by ROS suffer a greater loss of antimicrobial activity. These results, from an ecological point of view, are very interesting since the action of UVB light represents an adequate method for natural CFT degradation in water because it allows a decrease in the generation of multi-resistance to antibiotics by bacteria. Also, the presence of photolysis products does not show toxic effects on living organisms. [ABSTRACT FROM AUTHOR]