The synthesis of microbial products is mediated by either chromosomes, plasmids, or bacteriophages. The role of some of these products in the pathogenicity of the producing organisms is clearly understood, while the identity and role of others are still under investigation. This article reviews the effect of subinhibitory concentrations of antibiotics on the synthesis of various microbial products (virulence factors) by producing organisms. Synthesis in many cases is enhanced, whereas in others it is either decreased or unaffected. Highly selective inhibition of the production of some enzymes and toxins by certain antibiotics may be explained by the fact that the ribosomes synthesizing these proteins are peripherally localized and may be more sensitive to inhibition by low concentrations of antibiotics than are cytoplasmic ribosomes, although conclusive evidence for the synthesis of bacterial enzymes and toxins on membrane-bound ribosomes is lacking.