tion of not only the nuclear matrix preparation itself Vanadyl ribonucleoside and orthovanadate are but also the measurement of the activities associated commonly employed as inhibitors of ribonuclease and with this preparation. For example, we have recently protein phosphatase activities, respectively, in a varidescribed the association of protein kinase CK2 (also ety of tissue preparations. We have observed that the known as casein kinase 2) with the nuclear components presence of these agents in the tissue samples intersuch as chromatin and nuclear matrix where it underferes in the measurement of their protein content usgoes dynamic regulation in response to growth stimuli ing the Coomassie dye binding procedure. We have (8,9,11). An accurate analysis of the amount of protein demonstrated that this interference in the protein isolated in such preparations is essential, and since assay can be overcome by including H 2 O 2 at a final vanadyl ribonucleoside is routinely included in the meconcentration of 0.1% in the protein assay medium dium employed for preparation of the nuclear matrix, prior to the addition of the dye reagent. This results we describe here a procedure for eliminating the interin accurate measurements of the protein content in ference caused by this agent and orthovanadate in prothe tissue preparation containing vanadyl ribonucletein assays employing Coomassie dye binding method. oside or orthovanadate.