LY 127935 (6059-S), a new semi-synthetic beta-lactam antibiotic was tested simultaneously with 6 cephalosporins, 3 aminoglycosides, carbenicillin and ticarcillin against 398 clinical isolates of Gram-negative bacilli and Gram-positive cocci. Many of the organisms were selected for study because of known resistance to one or more of the clinically available antibiotics tested. Escherichia coli, Klebsiella, Serratia and Providencia were susceptible to LY 127935. Some resistant strains of Enterobacter, Proteus, Pseudomonas aeruginosa and Acinetobacter were also resistant to LY 127935, but many of the strains resistant to other antibiotics were susceptible to LY 127935. The activity of LY 127935 against Staphylococcus aureus, Streptococcus pneumoniae, Streptococcus pyogenes, Streptococcus viridans and Streptococcus bovis was similar to that of cephalexin and cephradine. LY 127935 was not active against methicillin-resistant S. aureus nor enterococcus.