The stability, potency, and preservative effectiveness of two dilutions of epoetin alfa containing a bacteriostatic diluent were studied. Epoetin alfa 10,000 units/mL in single-use vials was diluted 1:1 and 1:1.5 with bacteriostatic 0.9% sodium chloride injection. USP tests of preservative effectiveness were performed on samples from two batches each of the 1:1 and the 1:1.5 dilutions. Appearance assessment, Western blot analysis, radioimmunoassay, and bioassay were used to determine the stability or potency of samples from three batches of each dilution that were stored at 5 degrees C or at 30 degrees C for 12 weeks. Both batches of the 1:1.5 dilution (epoetin alfa 4,000 units/mL with 0.54% benzyl alcohol) met the USP criteria for preserved solutions, while one batch of the 1:1 dilution (epoetin alfa 5,000 units/mL with 0.45% benzyl alcohol) did not. Epoetin alfa 10,000 units/mL diluted either 1:1 or 1:1.5 with bacteriostatic 0.9% sodium chloride injection and stored at 5 degrees C or at 30 degrees C remained stable and potent for 12 weeks. The addition of 1.5 mL of bacteriostatic 0.9% sodium chloride injection to a vial containing 1 mL of epoetin alfa 10,000 units/mL makes a solution of epoetin alfa 4,000 units/mL that meets the USP criteria for preservative effectiveness and remains stable and potent for 12 weeks at 5 degrees C.