The in vitro activity of the novel beta-lactam antibiotic moxalactam against 430 bacterial isolates was determined by an agar dilution method. Moxalactam was highly active against Enterobacteriaceae including indole-positive Proteus and Providencia, all isolates being susceptible to 1.0 microgram/ml or less. The drug was somewhat less active against Gram-positive cocci and Pseudomonas aeruginosa, and enterococci were highly resistant. All Haemophilus influenzae isolates were inhibited by 0.06 microgram/ml or less. Activity against Bacteroides fragilis was good but wide variations of minimal inhibitory concentrations were noted. Moxalactam may become a useful alternative to the aminoglycosides in the treatment of serious infections caused by Gram-negative bacteria.