The in-vitro antibacterial activities of seventeen antimicrobial agents including ampicillin, amikacin, Augmentin, aztreonam, cefazolin, cefuroxime, cefotaxime, ceftizoxime, ceftriaxone, ciprofloxacin, cloxacillin, erythromycin, gentamicin, penicillin G, piperacillin and vancomycin were compared against 100 Gram-negative and Gram-positive strains isolated from blood culture specimens received at the King Abdulaziz University Hospital (KAUH), in Jeddah, Saudi Arabia. The antibacterial susceptibility was determined by the minimal inhibitory concentration (MIC), using an agar dilution method. Ciprofloxacin exhibited the greatest activity, inhibiting 90% of the tested strains (MIC90) at a concentration ranging from < 0.015-0.5 mg/L. Against cloxacillin resistant or susceptible strains of Staphylococcus aureus and coagulase-negative staphylococci, ciprofloxacin had similar activity with MIC90 of 0.2 mg/L. Salmonella typhi and salmonella species which were resistant to ampicillin and augmentin remained sensitive to ciprofloxacin (MIC90 < 0.015-0.125) mg/L.). Against gentamicin sensitive and resistant Pseudomonas aeruginosa and Pseudomonas species, ciprofloxacin MIC90 was 0.5 and 1 mg/L respectively. Aminoglycosides, third generation cephalosporins, aztreonam and antipseudomonal penicillins, on the other hand, showed high MIC90 well above the obtainable serum concentrations against Enterobacteriaceae and Pseudomonas species.