Aztreonam (SQ 26,776) is a new synthetic monocyclic beta-lactam antibiotic which is specifically active against aerobic gram-negative bacteria. High-pressure liquid chromatographic (HPLC) systems were developed for the quantitative analysis of aztreonam in human, monkey, rat, mouse, and rabbit sera and urine. The HPLC conditions employed for these analyses were a muBondapak C18 column, a mobile phase made up of 0.005 M tetrabutylammonium hydrogen sulfate at pH 3.0 and acetonitrile or methanol, UV detection at 293 nm, and a flow rate of 2.0 ml/min. For human sera and urine, the mobile phase was 80% 0.005 M tetrabutylammonium hydrogen sulfate-0.005M (NH4)2SO4 and 20% acetonitrile (vol/vol). For the range of sera and urine, HPLC analyses were shown to have excellent detector linearity of aztreonam over a concentration range of 1.0 mg/ml to 0.5 microgram/ml. Correlation coefficients for plots of aztreonam peak area versus its concentration were greater than or equal to 0.990. The detection limit of aztreonam was 1.0 micrograms/ml in sera and 5.0 micrograms/ml in urine. HPLC and microbiological assays of aztreonam in human sera and urine were in good agreement.