The compatibility of filgrastim with selected drugs during simulated Y-site injection was studied. Five-milliliter samples of filgrastim 30 micrograms/mL in 5% dextrose injection were combined with 5-mL samples of solutions of each of 97 other drugs in 5% dextrose injection at clinically used concentrations at 22 degrees C. Immediately after and one and four hours after the samples were combined, visual examinations were performed in fluorescent light with the unaided eye and with a high-intensity monodirectional light (Tyndall beam) to enhance the visualization of small particles and low-level turbidity. The turbidity of each drug combination was measured as well. Combinations yielding inconclusive results were subjected to particle sizing and counting. Most of the drugs tested were compatible with filgrastim 30 micrograms/mL during the observation period. However, 22 drugs showed various incompatibilities with filgrastim, including filament formation, particulate formation and precipitation, and color change. Filgrastim 30 micrograms/mL in 5% dextrose injection was compatible with 75 drugs for up to four hours at 22 degrees C; 22 drugs were not compatible with filgrastim.