Evaluation of antimicrobial agents in governed mainly by interaction between the pharmaceutical industry and regulatory authorities. The 1977 FDA guidelines have been setting the standards for more than a decade now. Basic principles of the 1977 guidelines remain valid, however changes in the definition of end-points of response, as measured by both clinical and microbiological criteria, have occurred. The new (draft) FDA guidelines and the 1989 guidelines of the British Society of Antimicrobial Chemotherapy are more consistent with contemporary concepts of treatment. In general, the differences in the requirements are minimal with a few exceptions, namely the requirements concerning blinding and assessment of clinical efficacy by site of infection and by organism in the FDA guidelines.