When it comes to bacterial disease, we are living in a state of false security. Antibiotics have indeed brought unprecedented health benefits, protection from and cure of bacterial diseases during the past 50 years. But there are ominous signs that the fortress and the defenses built on antibiotics are crumbling. They are crum bling because we wittingly or unwittingly created selective con ditions for the emergence of superior pathogens that can no longer be controlled by antibiotics. There are numerous warnings. After a long period of eclipse tuberculosis has now emerged as a serious threat unchecked by antibiotic treatment. Recent years have seen reports of cholera epidemics, of anthrax infections, of serious problems with Salmonella and even with E. coli, just to name a few. Mankind is in a race with microbial invaders. The challenge is to anticipate and respond to developments that affect the precarious balance between man and microbe. This will re quire new knowledge and it will take time for an effective appli cation of that knowledge.