The present study describes the effects of sodium molybdate on several physicochemical properties of the androgen receptor from the Dunning R3327 prostatic tumor. Molybdate was found to stabilize the steroid-binding activity of the receptor. Maximum binding activity was found with concentrations of 10 mM molybdate or greater. Density gradient and gel filtration analyses of the receptor revealed an 8.5-9.0S, 68A form (mol wt, approximately 265,000-275,000) in either the presence or absence of molybdate (20 mM) when determined under low ionic conditions. Under high ionic conditions (400 mM KCl), the receptor was maintained in a similar form (8.5-9.0S; 72-73A; mol wt, approximately 275,000-295,000) in the presence of molybdate; however, it disaggregated to a smaller 4.4S, 61 A form (mol wt, approximately 120,000) in the absence of molybdate. Affinity labeling of the receptor with 17 beta-[(bromoacetyl)oxy]-[1,2,4,5,6,7,16,17-3H8]5 alpha-androstan-3-one showed a mol wt of 118,000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and fluorography. Molybdate inhibited the salt-induced transformation of the receptor to a DNA-binding state, but did not inhibit the DNA binding of the receptor transformed previously in the absence of molybdate. These studies suggest that sodium molybdate stabilizes the steroid-binding activity of the androgen receptor in an aggregated nontransformed state.