Serum bactericidal activity was determined against 10 strains each of Pseudomonas aeruginosa and Staphylococcus aureus in serum from volunteers 1 and 6 h after intravenous infusion of cefotaxime, tobramycin, and the combination; or of moxalactam, tobramycin, and the combination. High serum bactericidal activity against P. aeruginosa was found significantly more frequently with moxalactam plus tobramycin than with cefotaxime, moxalactam, and tobramycin alone or with cefotaxime plus tobramycin.Moxalactam (1-oxa-/l-lactam) and cefotaxime are new antibiotics with increased activity against Enterobacteriaceae relative to older penicillins and cephalosporins (2,9,(12)(13)(14)(15). In addition, both drugs have in vitro activity against Pseudomonas aeruginosa (2,9,12,13,15). Both of these agents have in vitro activity against Staphylococcus aureus, but this activity may not be as great as that of older antibiotics of similar composition (2,9,12,15). Cephalosporins and penicillins are used frequently in combination with aminoglycoside antibiotics in the treatment of gram-negative rod bacteremia (4), and the synergistic activity obtained with these combinations has been related to improved outcome in severely neutropenic bacteremic patients (6)