Prostate-specific antigen (PSA) is secreted both by normal epithelial prostatic cells and cell of prostatic carcinoma (PC). No parallelism exists between the degree of PC differentiation and the type of PSA secretion. PSA concentration in the peripheral blood not always corresponds to the intensity of its immunoreactivity on the tissue level. Amount of PSA in the blood depends on the stroma vascularisation and number of cells contacting with the organ stroma. A decrease of PSA in the peripheral blood due to therapy may be combined with high intensity of its synthesis in tumor cells. PSA immunohistochemistry may be recommended as a method of PC clinical course monitoring and dynamics of its changes in the course of carcinoma therapy.