A process for preparing penicillin G procaine comprises the steps of introducing inactive ingredients into a first mixing tank and sterilizing the mixture of inactive ingredients in the first mixing tank. A procaine hydrochloride is dissolved in a second mixing tank, and the contents of the second mixing tank are added to the first mixing tank through a sterile filter. A solution of penicillin G potassium is introduced to the first mixing tank through a sterile filter, and the contents of the first mixing tank are mixed so that the procaine hydrochloride reacts with the penicillin G potassium forming penicillin G procaine. The resulting suspension is transferred through a homogenizer to a holding tank and penicillin G procaine is filled into a container.