Microbial genetics and molecular cloning now permit us to routinely isolate specific genes from a variety of microbial pathogens. Obviously not all genes from pathogenic microorganisms play a role in pathogenicity or virulence. Just as Koch's postulates were formulated to identify the causal relationship between an organism and a specific disease, the notion is presented here that a form of molecular Koch's postulates is needed when examining the potential role of genes and their products in the pathogenesis of infection and disease.