A method for transferring and mixing a pharmaceutical mixture utilizing a by-pass cartridge and a syringe. After mixing the dry component and the diluent in the by-pass cartridge, the mixture can be passed to a syringe which includes a syringe plunger having a fluid passageway therein with a hydrophobic membrane covering the passageway. The mixture can be transferred back and forth between the by-pass cartridge and the syringe through a small bore to achieve mixing, particularly in the case of microspheres held in suspension.