In the course of our search for derivatives with potent antibacterial activity and low cytotoxicity, we have studied the relationships among the structure, antibacterial activity and cytotoxicity of thiazoloquinolone and thiazetoquinolone derivatives (the term quinolone as used in this paper includes the quinolone, 1,8-naphthyridine and pyridopyrimidine nuclei). The antibacterial activities and cytotoxicity of these derivatives were compared with those of norfloxacin, ofloxacin, enoxacin and ciprofloxacin. The antibacterial activities of the thiazoloquinolone derivatives were more potent than those of the dihyrothiazoloquinolone derivatives, and comparable to that of ciprofloxacin. All of the thiazoloquinolone derivatives were highly cytotoxic against mammalian cells, but some of the dihyrothiazoloquinolone derivatives were less cytotoxic, being comparable in cytotoxicity to the reference drugs. The thiazetoquinolone derivatives were less cytotoxic than the thiazoloquinolone derivatives, and one of them, 6-fluoro-1-methyl-4-oxo-7-(1-piperazinyl)-4H-[1,3]thiazeto[3,2-a] quinoline-3-carboxylic acid, showed the most potent antibacterial activity of all compounds tested in this study, as well as a very low cytotoxicity. The antibacterial activity and cytotoxicity of this compound were similar to that of ciprofloxacin.