By in vivo labeling with [75Se]selenite and separation of the proteins in the tissue homogenates by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), several selenium-containing proteins or protein subunits were detected in rat tissues (liver, lung, spleen and prostate). Their distribution among the cell components was investigated after fractionation by means of differential centrifugation. The selenium-containing proteins in the 16 kDa range were found to be mainly membrane-bound. By two-dimensional electrophoresis they were resolved into three labeled spots, two with the same relative molecular mass and pI values of about 4.8 and 5.0 and the third with a slightly lower molecular mass and a pI of 4.8. For further investigation they were concentrated and separated from the other labeled compounds by SDS-PAGE using preparative flow-through electrophoresis.