The acute toxicity of sodium fosfomycin (FOM-Na), a new antibiotic in ICR mice and Wistar rats has been investigated. The LD50 values in mice were: 1,230 (male), 1,225 (female) mg/kg by i.v.; 2,175 (m), 2,467 (f) mg/kg by i.p.; 2,625 (m), 2,662 (f) mg/kg by i.m.; 5,100 (m), 6,150 (f) mg/kg by s.c. and 8,020 (m), 7,300 (f) mg/kg by p.o. The LD50 values in rats were: 1,650 (m), 1,560 (f) mg/kg by i.v.; 2,060 (m), 2,000 (f) mg/kg by i.p.; 2,630 (m), 2,460 (f) mg/kg by i.m.; 5,100 (m), 4,320 (f) mg/kg by s.c. and 4,700 (m), 4,550 (f) mg/kg by p.o. Animals dosed only i.v. died within 2 minutes. Signs of toxicity in mice or rats given FOM-Na were similar and included motor activity depression, reduced respiration and occasionally tremors. Surviving mice or rats given FOM-Na developed no pathological changes of the drug specificity.