Immunohistochemical identification of the most prevalent type of neuroendocrine (NE) cells in the human prostate gland can be made with polyclonal antisera against human thyroid-stimulating hormone (TSH). A TSH-like peptide was characterized by analysis of prostatic tissue homogenates with sodium dodecyl sulfate-polyacrylamide gel (SDS-PAGE) electrophoresis followed by immunoblotting. A single protein band, with an apparent mass of about 32 kDa after reduction, was identified both with polyclonal antisera against human TSH and with a polyclonal antiserum raised against a synthetic peptide corresponding to the carboxyterminal part of the beta-subunit of human TSH. The TSH-like prostatic peptide identified here is, on the basis of its molecular mass and absence of immunoreactivity with an antiserum raised against a synthetic peptide representing the mid-portion of the beta-subunit of TSH, not identical with the pituitary beta-subunit of TSH. On the other hand, this 32 kDa prostatic peptide may have certain structural elements in common with the pituitary beta-subunit of TSH, since it is recognized both with polyclonal antisera against TSH and with an antiserum against the carboxyterminal part of the beta-subunit of TSH.