Staphylococcus aureus, which has exceptional ability to cause disease in man, has responded to the introductions of antimicrobial agents by the selection and emergence of antibiotic-resistant strains. There have been cyclical changes in the prevalence of these strains in nosocomial infections. Other Gram-positive species that give rise to concern, for related reasons, are Staph. epidermidis and other coagulase-negative staphylococci, Streptococcus faecalis and the JK group of corynebacteria. These problems are discussed to provide a background for the contributions that follow.