Antibodies raised against prostatic specific antigen (PSA) and immunoglobulin binding factor (IgBF) of human seminal plasma (SP) were used to localize the antigens in various tissues by Western blot. Both antigens were found only in the prostate, including benign prostatic hypertrophy and prostatic adenocarcinoma. The polyclonal anti-PSA antibodies stained five prostatic protein bands with estimated M(r) values of 10, 14, 22, 25, and 33 kD, whereas anti-IgBF antibodies stained a single 16-kD protein. No cross-reaction occurred between the two antibodies. When anti-PSA antibodies were used an additional protein with an estimated M(r) of 35 kD was detected in the extract of benign prostatic hypertrophy, but not with normal prostate or prostatic cancer. When SP and prostatic proteins were analyzed by SDS-PAGE under nonreducing condition and immunoblot with both antibodies, immunoreactive proteins with estimated M(r) of 125 and 140 kD, respectively, were stained, suggesting that both factors may be produced as an aggregated precursor molecule. Since IgBF was found only in the prostate, this component may be useful as a marker of prostatic tissue.