A new antibiotic of cephamycin group, cefminox (CMNX, MT-141) was studied both fundamentally and clinically in the field of pediatrics. The minimum inhibitory concentrations (MIC) of CMNX for clinical isolates including 24 strains of S. aureus, 15 strains of S. pyogenes, 21 strains of H. influenzae, 24 strains of E. coli, 22 strains of K. pneumoniae and 22 strains of P. mirabilis were determined and compared to those of cefmetazole (CMZ), latamoxef (LMOX), cefotaxime (CTX), cefoperazone (CPZ) and cefazolin (CEZ). The MIC80 (80% MIC) values of CMNX for H. influenzae, E. coli, K. pneumoniae and P. mirabilis were 1.56, 1.56, 0.39 and 1.56 micrograms/ml, respectively. When compared to antibacterial activities of the control drugs, the activity of CMNX was inferior to those of CTX and LMOX but superior to those of CMZ and CEZ. On the other hand, MIC80 values of CMNX for S. pyogenes and S. aureus were 6.25 and 12.5 micrograms/ml, the activities being inferior to all of CMZ, CTX, LMOX, CPZ and CEZ used as the control drugs. In 3 pediatric patients of 9 to 12 years old, 20 mg/kg of CMNX was given intravenously as one shot and serum and urinary concentrations were determined. The mean serum concentrations in these 3 cases were 124 micrograms/ml, 102 micrograms/ml, 74.0 micrograms/ml, 47.9 micrograms/ml, 20.4 micrograms/ml, 9.2 micrograms/ml and 4.3 micrograms/ml at 1/4, 1/2, 1, 2, 4, 6 and 8 hours, respectively, with a half-life of 1.83 hours. The mean urinary concentrations were 1,968 micrograms/ml at 0 approximately 2 hours, 1,205 micrograms/ml at 2 approximately 4 hours, 761 micrograms/ml at 4 approximately 6 hours and 409 micrograms/ml at 6 approximately 8 hours, with 65.4% of the drug dosed recovered from the urine within the first 8 hours on an average. CMNX was used in the treatment of 22 clinical cases including 3 cases of acute purulent tonsillitis, 3 cases of acute bronchitis, 9 cases of acute pneumonia, 5 cases of acute pyelonephritis and 2 cases of acute enteritis. Clinical results in 20 cases excluded of 2 cases of Mycoplasma pneumonia were rated as excellent in 19 cases and as good in 1 case, with an efficacy rate being 100% taking excellent and good cases as effective cases. Bacteriological results for 5 strains of H. influenzae, 1 strain of H. parainfluenzae, 5 strains of E. coli, 2 strains of K. oxytoca and 1 strain of S. pneumoniae revealed that disappearance was obtained for all strains but 1 strain of P. aeruginosa which persisted.(ABSTRACT TRUNCATED AT 400 WORDS)