C31G is a specific formulation which contains equal molar concentrations of alkyl N-betaine and alkyl N,N-dimethylamine oxide. Vaginal suppositories containing 500 mg of C31G, this potent antimicrobial substance, were prepared by the fusion method in a variety of Suppocire(R) and Witepsol(R) bases with different melting points and hydroxyl values. In vitro release and diffusion characteristics of C31G from different suppository bases were investigated using two different systems. The release from suppositories was determined by using a system without a membrane and the diffusion rate of the released agent was determined through a semipermeable dialyzing tubing. Diffusion kinetics from suppositories were evaluated in terms of the apparent dialytic rate constants using the equation developed by Davis et al.