The in-vitro antibacterial activity of SQ 26.776 (SQ), a new β -lactam antibiotic belonging to the recently discovered group of monobactams, was studied using agar dilution and broth dilution techniques for MIC-determinations. SQ was found to have little or no effect on pneumococci, staphylococci and β -haemolytic streptococci, group A. Its activity against Escherichia coli . Kiebsiella. Proteus mirabilis , indole-positive strains of Proteus spp., Enterobacter . and Citrobacter was excellent with MICs normally below 0.5 mg/l. The activity against Pseudomonas and Acinetobacter was characterized by inhibitory concentrations between 4 and 32 mg/l. Comparing SQ with ampicillin and cephalothin, against the Gram-negative species tested, the activity of the two latter were clearly inferior. Gentamicin had an activity similar to that of SQ against most Gram-negatives but was superior against Pseudomonas spp. and Acinetobacter spp. The anti-pseudomonas activity of carbenicillin was not as good as that of SQ. The inhibitory concentrations of SQ were only marginally affected by the technique used or by the size of the bacterial inoculum. Thus, agar dilution MICs were similar to broth dilution MICs and increases of the inoculum size by a factor 100 did not in any case cause larger than fourfold increases of the inhibitory concentrations. With its selective Gram-negative antibacterial spectrum, SQ seems to offer a valuable addition to the modern therapeutic armentarium.