The compatibility of aztreonam injection with selected other drugs during simulated Y-site injection was evaluated. A 5-mL sample of aztreonam 40 mg/mL in 5% dextrose injection was combined with 5 mL of each of 100 other drugs including antineoplastics, anti-infectives, and supportive care drugs in 5% dextrose injection or 0.9% sodium chloride injection. The combinations were stored under fluorescent light at 23 degrees C. The combinations were evaluated initially and at one and four hours after preparation by examination with the unaided eye and with a high-intensity light beam, turbidity measurement, and particle sizing and counting. Most of the test drugs were compatible with aztreonam 40 mg/mL for the observation period. However, aztreonam combinations with 12 drugs exhibited incompatibilities including turbidity, particulate formation, precipitation, and color change. These drugs were acyclovir sodium, amphotericin B, chlorpromazine hydrochloride, daunorubicin hydrochloride, ganciclovir sodium, lorazepam, metronidazole, miconazole, mitomycin, mitoxantrone hydrochloride, prochlorperazine edisylate, and streptozocin. Aztreonam 40 mg/mL in 5% dextrose injection was compatible with most of the drugs tested for up to four hours at 23 degrees C. However, 12 drugs exhibited incompatibility with aztreonam.