After labeling of rats in vivo with 75Se and protein separation by sodium dodecyl sulfatepolyacrylamide gel electrophoresis more than 25 Se-containing bands could be distinguished.Of those proteins which were detected only in certain compartments and might therefore have tissue-specific functions, two were chosen for detailed investigation.A 15 kDa-protein was found in the prostatic epithelium where it accounted for about two thirds of the protein-bound 75Se. It was mainly present in the cytosol but was not released into the prostatic secretion. After gel chromatography it was found in the fraction which contained proteins with molecular masses of about 300 kDa. Using two-dimensional electrophoresis a plvalue of about 4. 5 was determined.In the testis a specific Se-containing 34 kDa-protein was observed which appeared after the onset of puberty. It was localized in the spermatid nuclei where it contained about 80% of the Se tracer present and was found to be bound to the DNA. After extraction it partly disintegrated into a 20 kDa-protein.Both compounds contain Se in the form of selenocysteine. The fact that their formation had priority over that of glutathione peroxidase during insufficient Se intake is an indication of their biological significance. Special interest in the prostatic epithelial selenoprotein derives from a possible inverse relationship between the Se status and the incidence of prostate cancer observed in epidemiological studies, whereas with the 34 kDa-selenoprotein its appearance during the condensation phase of the spermatid nuclei might suggest its participation in some processes of sperm maturation After labeling of rats in vivo with 75 Se and protein separation by sodium dodecyl sulfate polyacrylamide gel electrophoresis more than 25 Se-containing bands could be distinguished. Of those proteins which were detected only in certain compartments and might therefore have tissue-specific functions, two were chosen for detailed investigation. A 15 kDa-protein was found in the prostatic epithelium where it accounted for about two thirds of the protein-bound 75Se. It was mainly present in the cytosol but was not released into the prostatic secretion. After gel chromatography it was found in the fraction which contained proteins with molecular masses of about 300 kDa. Using two-dimensional electrophoresis a plvalue of about 4.5 was determined in the testis a specific Se-containing 34 kDa-protein was observed which had after the onset of puberty. It was localized in the spermatid nuclei where it contained about 80% of the Se tracer present and was found to be bound to the DNA. After ext The fact that their formation had priority over that of glutathione peroxidase during insufficient Se intake is an indication of their biological significance. Special interest in the prostatic epithelial selenoprotein derives from a possible inverse relationship between the Se status and the incidence of prostate cancer observed in epidemiological studies, but with the 34 kDa-selenoprotein its appearance during the condensation phase of the spermatid nuclei might suggest its participation in some processes of sperm maturation