(S)-10-[(S)-(8-Amino-6-azaspiro[3,4]octan-6-yl)]-9-fluoro-2, 3-dihydro-3-methyl-7-oxo-7H-pyrido[1,2,3-de][1,4]benzoxazine-6-carboxyli c acid hemihydrate (CAS 151390-79-3, DV-7751a) a new quinolone antibacterial agent, was examined for LD50 value, phototoxicity and convulsion inducing potential in laboratory animals. A single oral administration of DV-7751a induced soft stool in rats at 1000 and 2000 mg/ kg and in monkeys at 250 mg/kg and vomiting in monkeys at 500 mg/kg or more. A single intravenous administration caused a decrease in locomotor activity, respiratory depression, convulsion, pulmonary edema and death in rats and mice. The LD50 values with oral administration were more than 2000 mg/ kg for rats and mice and more than 250 mg/kg for monkeys, and those with intravenous administration were 164.3 mg/kg for rats of both sexes at an injection rate of 2 ml/min, 118.8 mg/kg for male rats and 104 to 125 mg/kg for female rats at 0.5 ml/min, and 184.7 mg/kg for male mice and 187.4 mg/kg for female mice. DV-7751a showed very weak phototoxicity in mice after single oral administration of 600 mg/kg, followed by UVA irradiation, but no convulsion after oral administration of 200 or 1000 mg/kg in combination with 4-biphenylacetic acid at 400 mg/kg.