Pepsinogen, the inactive precursor of pepsin belongs to the group of aspartic proteinases, is synthesized in the chief cells of gastric glands. Seven electrophoretically separable pepsinogens exist in human gastric mucosa. One group (Pg1-Pg5), called PGA or group I pepsinogen, is characterized by electrophoretically faster migration and localization in the fundus and body of the stomach; the second group (Pg6-Pg7), termed pepsinogen C, is localized in the whole stomach. PGCs are found in prostate and in seminal fluid whereas PGAs are not. the absence of detectable immunologic crossreactivity between the pepsinogens A and C results from divergent evolution of sequences located on the surface of the zymogens in contrast to the strongly conserved active site regions located within the binding cleft of the enzymes, inaccessible for antigenic recognition.