17 beta-estradiol induces the synthesis of massive amounts of the hepatic mRNA encoding the Xenopus laevis egg yolk precursor protein, vitellogenin. Vitellogenin mRNA exhibits a half life of approx. 500 h when 17 beta-estradiol is present, and 16 h after removal of 17 beta-estradiol from the culture medium. We recently reported that Xenopus liver contains a protein, which is induced by 17 beta-estradiol and binds with a high degree of specificity to a binding site in a segment of the 3'-untranslated region (3'-UTR) of vitellogenin mRNA implicated in 17 beta-estradiol stabilization of vitellogenin mRNA. To determine if this mRNA binding protein was specific to this system, or if it was present elsewhere, and regulated by other steroids, we examined the tissue distribution and androgen regulation of this protein. Substantial amounts of the vitellogenin 3'-UTR binding protein were found in several Xenopus tissues including testis, ovary and muscle. In the absence of hormone treatment, lung and intestine contained minimal levels of the mRNA binding protein. Testosterone administration induced the vitellogenin 3'-UTR RNA binding protein in several tissues. Additionally, we found a homologous mRNA binding protein in MCF-7, human breast cancer cells. Although the MCF-7 cell protein was not induced by 17 beta-estradiol, the MCF-7 cell mRNA binding protein appears to be closely related to the Xenopus protein since: (i) the human and Xenopus proteins elicit gel shifted bands with the same electrophoretic mobility using the vitellogenin mRNA 3'-UTR binding site; (ii) The human and Xenopus proteins exhibit similar binding specificity for the vitellogenin 3'-UTR RNA binding site; and (iii) RNA from MCF-7 cells is at least as effective as RNA from control male Xenopus liver in blocking the binding of the Xenopus and human proteins to the vitellogenin mRNA 3'-UTR binding site. Its broad tissue distribution and regulation by both 17 beta-estradiol and testosterone suggests that this mRNA binding protein may play a significant role in steroid hormone regulation of mRNA metabolism in many vertebrate cells.