Ceftriaxone (rocefin), a 3rd generation cephalosporin with prolonged action, was highly efficient (ED50 0.12 to 0.38 mg/mouse) against experimental plague in albino mice infected either by Fra+ or Fra- strains of the plague microbe. The daily doses of the antibiotic (0.5-1.0-2.0 mg/mouse) provided 85-100 per cent survival of the animals. All the strains (15) of the plague microbe isolated from different natural foci and different objects were highly sensitive to ceftriaxone. The MIC was 0.02 to 0.05 microgram/ml.