Proteolytic enzymes from the serine endopeptidase family found in normal blood and urine. Specifically, kallikreins are potent vasodilators and hypotensives and increase vascular permeability and affect smooth muscle. They act as infertility agents in men. Three forms are recognized, plasma kallikrein (EC 3.4.21.34), TISSUE KALLIKREIN (EC 3.4.21.35), and prostate-specific antigen (EC 3.4.21.77).