The authors have carried out the laboratory and clinical studies of cefroxadine (CXD), and obtained the following results. The antibacterial activities of CXD were measured by plate dilution method on 26 clinical isolates of S. aureus, E. coli and K. pneumoniae. CXD inhibited the growth of all strains of S. aureus at concentrations less than 6.25 microgram/ml, the peak of activity distribution was obtained at 3.13 microgram/ml with an inoculum size of 10(6) cells/ml. And the p eak sensitivity distribution of E. coli was obtained at 6.25 microgram/ml. The growth of all strains of K. pneumoniae was inhibited at concentrations of less than 25 microgram/ml. Phagocytosis was determined by QUIE'S method. In the presence of CXD, phagocytosis of human PMNs was not enhanced to E. coli and K. pneumoniae. For pharmacokinetic study, CXD was given orally at a single dose of 10 mg/kg to 3 children before and after meals. The serum levels of CXD on fasting were 14.2 microgram/ml, 11.0 microgram/ml, 4.0 microgram/ml and 0.57 microgram/ml at 0.5, 1, 2. 4 hours after administration respectively, and the level at 6 hours was not detectable. Half-life was 0.65 hours. The serum levels of CXD after meals were 3.9 microgram/ml, 5.3 microgram/ml, 5.3 microgram/ml, 2.4 microgram/ml and 0.42 microgram/ml at 0.5, 1, 2, 4, 6 hours after administration respectively, but at 8 hours it was not detectable. Half-life was 0.95 hours. The 8-hour urinary excretion rates on fasting and non fasting were 89.4%, 89.0% respectively. CXD was given to 31 cases with tonsillitis, 4 with bronchitis, 1 with impetigo, 3 with cervical lymphadenitis, 7 with U.T.I, totalling 46. A daily dose of CXD 400 approximately 1,500 mg was given for 4 approximately 14 days. Clinical results obtained were good and excellent responses in 43/46 (93.5%) cases. No side effects were observed except for 1 case with elevation of GOT, 2 cases with elevation of GOT and GPT and 1 case with eosinophilia.