The properties, distribution, and biological functions of several proteases from the plasma membrane of lymphoid cells are reviewed: dipeptidyl peptidase IV, neutral endopeptidase, aminopeptidases A and N, and a new protease of the family of adamalysins. these enzymes (designated as ectopeptidases) are integral membrane proteins whose molecules are mainly located extracellularly. Their functions involve proteolysis on the cell surface: the formation and inactivation of regulatory peptides and growth factors, as well as modification of cell surface proteins. The biological significance of a partial proteolysis of the plasma membrane proteins and the resulting soluble isoforms are discussed. An analysis of the data suggests that ectopeptidases from lymphoid cells are elements of the sensory system of the cell and are involved in the regulation of its physiological response to external factors and in the coordination of cell-cell interactions.