The human sera often contain substances which interfere in the course of immunochemical reactions. lt is essential for the control sera used to determine concentrations of antigen of the prostate not to contain such ingredients. The aim of this study was to test the control sera, three producers, containing low (N) and high (P) concentrations of prostate antigen (tPSA). As a factor blocking the interfering actions, the TRU Bloc Meridian Life Science, lnc. reagent was used which contains antibodies against mouse protein and rheumatoid factor. The sera with a low concentration of tPSA demonstrated the presence of interfering substances.