Laboratory studies have been performed on ceftezole (CTZ), a newly developed cephalosporin antibiotic, and this drug has been applied clinically to various infections of respiratory organs. Results obtained were as follows. 1) Antibacterial activity : Minimum inhibitory concentrations of CTZ were determined by means of standard method of the Japan Society of Chemotherapy on 519 strains in total including 22 strains preserved in our department as standard and 497 strains isolated from various clinical materials (Staph. aureus 64, β-Streptococcus 32, Salmonella 16, Shigella 15, E. coli 64, Kl. aerogenes 64, Ent. aerogenes 32, Ent. cloacae 32, Serratia marcescens 64, Pr. vulgaris 20, Pr. mirabilis 30 and Ps. aeruginosa 64). Compared these results with those of cefazolin, antibacterial activity of two drugs was almost the same. 2) Distribution in organs of rat : CTZ was injected intramuscularly to male Wistar rats at a dose of 20 mg/kg, and the drug concentrations in various organs were measured. Peak was attained in 30 minutes, and levels were arranged in order of kidney > serum > lung > liver. Ratios of organs level to serum level were kidney 5.017, lung 0.569 and liver 0.353. 3) Blood level in human : CTZ was injected intramuscularly by drip infusion for 2 hours at a dose of 3 g with 5% glucose, and blood concentrations of the drug were measured. Peak value of 78-85 μg/ml was shown at termination of drip infusion, and no blood activity was noticed 6 hours after completion of the infusion. Half life time was about 30 minutes. 4) Level in sputum : CTZ was administered similarly to 3) to the cases of bronchiectasis expectorating purulent sputum, and CTZ activity of 0.8 μg/ml was noticed in sputum. Ratio of sputum level to maximum blood level was 0.01. 5) Clinical results of respiratory infections : CTZ was injected intravenously by drip infusion at a dose of 3 g once daily to 6 cases of respiratory infections. Results obtained were excellent in 3 cases of bacterial pneumonia, fair in 1 case of lung abscess, and failure in 2 cases of bronchiectasis. 6) Side effect : Slight elevation of GOT and GPT was observed in 1 case out of the above 6 cases, though the values were normalized after interruption of the drug administration. No abnormality was noticed in other subjective symptoms and hemato-biochemical findings.