Important progress in the elucidation of the mechanisms influencing bacterial pathogenicity has recently been made through the introduction of modem genetic techniques. Molecular cloning allows the isolation of genes for pheno- types that epidemiological surveys have suggested play an important role in pathogenesis. The structural analysis of determinants for pathogenic traits can lead to the identifica- tion not only of the primary sequence but also of the possi- ble secondary and tertiary structures for important viru- lence factors such as toxins and adhesins. From these data, the prediction of antigenic domains suitable for the devel- opment of new vaccines appears to be feasible. The regula- tion of virulence determinants by endogenous and exoge- nous factors can be more clearly understood through the functional analysis of the cloned virulence genes. This volume surveys representative virulence properties of gram-positive and gram-negative bacteria to which the genetic approach has been successfully applied. The exam- ples described here include important bacterial toxins (e.g., diphtheria toxin, cholera toxin, toxic shock syndrome toxin, hemolysins), adhesion structures from E. coli and Neisseria gonorrhoeae, and factors supporting iron uptake, serum resistance, and invasiveness in a variety of bacteria. Both the present state and the possible futural develop- ments of these systems are described.