Summary The proteolytic enzymes of lactococci are of eminent importance for milk fermentations. By the combined action of proteinases and peptidases milk protein is degraded to peptides and amino acids which are required for cell growth and contribute to the organoleptic properties of the foods. The importance of the proteolytic system for dairy product quality has resulted in an increased fundamental research of the enzymes and genes involved. Proteinase plasmids have been identified and plasmid stability problems offered an explanation for the apparent instability of proteolysis in certain strains of lactococci. Chromosomal integration has recently been used to stably anchor the proteinase genes in the chromosome of Lactococcus lactis. The structural proteinase genes of a number of strains have been cloned and sequenced, and some of the properties of the enzymes they specify will be discussed. The product of a second gene is necessary for the activation of the proteinase, a proteinase maturation process that is unique in the bacterial world.