A new semisynthetic 1-oxa-beta-lactam derivative, 6059-S, was evaluated for its safety and efficacy in children. Twenty-five patients were treated with 10 to 274 mg/kg per day of 6059-S by intravenous administrations. The diagnosis of the patients were acute pharyngitis (2), acute bronchitis (2), pneumonia (4), pertussis (4), acute enterocolitis (2), recurrent urinary tract infection (2), suspected septicemia (3), and acute purulent meningitis (1); and the remaining 5 patients were considered to have nonbacterial infections. The pathogens recovered were Streptococcus pneumoniae (1), Haemophilus influenzae (4), Haemophilus parainfluenzae (1), Enterobacter cloacae (1), Enterobacter aerogenes (1), Proteus morganii (1), Psuedomonas aeruginosa (2) and Salmonella typhimurium (1). All the patients of bacterial infections were cured after the 6059-S therapy. However, Pseudomonas aeruginosa and Salmonella typhimurium were not eradicated after the 6059-S therapy, and the rate of bacterial disappearance was 75%. Diarrhea (3), precordial pain (2, only in cases with high-dose therapy), transient elevation of GOT and GPT (2), and transient eosinophilia (2) were found to be associated with the 6059-S therapy. However, no severe adverse reactions were encountered. Half life of the serum 6059-S level was 1.34 +/- 0.16 hours. CSF concentrations in a case with Haemophilus influenzae meningitis ranged 4.0 to 9.7 mcg/ml after an intravenous injection of 34.3 to 75 mg/kg of 6059-S. From the present study, 6059-S appears to be a safe and effective antibiotic when used in children with susceptible bacterial infections. It remains to be further determined whether 6059-S is superior to ABPC in the treatment of Haemophilus influenzae meningitis.