The compatibility of lyophilized thiotepa injection with selected other drugs during simulated Y-site injection was evaluated. Five-milliliter samples of thiotepa (lyophilized) 1 mg/mL in 5% dextrose injection were combined with 5 mL 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 at room temperature (approximately 23 degrees C) under constant fluorescent light. Visual examinations were performed immediately and at one and four hours with the unaided eye and, if there was no obvious incompatibility, with a high-intensity mono-directional light beam to enhance visualization of small particles and low-level turbidity. The turbidity of each combination was measured as well. Particle sizing and counting were performed on selected solutions. Most of the test drugs were compatible with thiotepa 1 mg/mL during the observation period. Two drugs exhibited incompatibilities with thiotepa. The thiotepa-cisplatin combination developed turbidity in four hours, and the thiotepa-minocycline hydrochloride combination developed a bright yellow-green discoloration in one hour. All the test drugs except cisplatin and minocycline hydrochloride were compatible with thiotepa 1 mg/mL (prepared from the lyophilized formulation) for at least four hours at room temperature.