The nephrotoxicity, pharmacokinetic and therapeutic activity of fosfomycin were investigated in female wistar-rats. Measures of nephrotoxicity were urinary excretion of tubular cells and of the enzymes MDH, LDH, and GOT. Histological investigations and estimation of serum urea concentration and proteinuria were also evaluated. The doses of 500, 1000, 2000, 3000, and 5000 mg/kg/d were administered in 9 single doses with 12 hours interval. The lowest dose which induced a significantly increased tubular cell excretion was 1000 mg/kg/d and therefore in the same range as the tubulotoxic threshold doses of cephalosporins. Chemotherapy of the chronic estrogen induced pyelonephritis revealed equally favourable results for fosfomycin and cefuroxim at dosages of 2 X 150 mg/kg/d. The pharmacokinetics of fosfomycin at a single dose of 150 mg/kg/d were equivalent to those of cefuroxim. These animal experiments showed fosfomycin to be of value as a therapeutic alternative to cephalosporin antibiotics.