12 cases of severe pulmonary infection were treated with ceftizoxime, one of cepha-losporins of the third generation. Clinical observation and pharmacokinetics were carried out. The result showed that intravenous infusion was the best route of drug administration. It had the advantages of high serum concentration and prolonged maintenance of serum level. 1 gram of ceftizoxi-me, two times a day, was enough for most pathogens except Pseudomonas aeruginosa.