Prostate specific antigen (PSA) is a kallicrein produced by the prostate, among several different substances, as part of its function as an organ of the reproductive system. Semen acts as a protease denaturing semenogelines, the procoagulant proteins of the semen produced by the seminal vesicle, which similar to other proteases has a potential ability to metabolize any protein. For such destructive function, nature takes precaution so that PSA-protease has a short activity period and a series of fractions that are cleaved or inactive, which explain altogether the percentage of the total antigen and the free fraction in the plasma of the healthy and the sick patient. PSA can be plainly divided in two: active and non-active or free and complexed. In the future there is hope that the PSA as a tumor marker can increase its sensitivity and specificity by the use of fractions that relate mathematically, as well as by the use of other antigens such as the prostate specific membrane antigen (PSMA) and the prostate stem cell antigen (PSCA).