Nuclear protein matrix was isolated from guinea pig seminal vesicle epithelium and liver. The two matrices were similar in fine structure as seen by transmission electron microscopy, in protein electropherograms, and in percent composition relative to protein, DNA, and RNA. Scanning electron microscopy was used to examine intact seminal vesicle nuclei, nuclei after treatment with Triton X-100 and DNAse I, and purified nuclear matrix. The matrix surface presented a 'porous' appearance by both scanning and transmission electron microscopy. The matrices of liver and seminal vesicle epithelium (SVE) and the intact nuclei of SVE were assayed for specific binding of free synthetic androgen, 17 alpha-methyltrienolone (R1881). Saturable specific binding was demonstrable for seminal vesicle matrix but not for liver matrix. Maximal binding of androgen occurred at a concentration of approximately 12 nM and was demonstrated to be 1.34 +/- 0.22 pmol of R1881 per mg of seminal vesicle matrix protein; the Kd was approximately 8 nM. The binding of labeled R1881 to matrix could be inhibited with low concentrations of unlabeled androgens, but not with estrogens or other steroids. Our data indicate that the binding of androgen to matrix could account for at least 21% of the binding to intact nuclei.