Within the broad range of activities in pharmaceutical research, the technological development and analytical evaluation of a new drug formulation represent only one step. The pharmaceutical development of three oral formulations of a combination product containing N1-(4,5-dimethyl-2-oxazolyl)-sulfanilamide (sulfamoxole) and 2,4-diamino-5-(3,4,5-trimethoxy-benzyl)-pyrimidine (trimethoprim) in a 5:1 ratio (investigational drug CN 3123; Nevin; Supristol), which was based on the physico-chemical characteristics of the 2 active substances, is presented. The various chemical and physical tests conducted with the drug formulations are described. The pharmaceutical or in-vitro availabilities (dissolution rate) of the active ingredients were determined by way of release-rate profiles, both of the active ingredients alone and in their final formulations. Also presented are the results of plasma level determinations following oral administration of film-coated tablets and a suspension. Finally, preliminary results of extensive stability tests with the three drug formulations are discussed.