Specific antibodies to Xenopus laevis bone morphogenetic protein-4 (xBMP-4) were raised by immunizing rabbits with a fusion protein of bacterial beta-galactosidase and xBMP-4. The antibodies were used to detect xBMPs expressed in mammalian cells by Western blotting. The antibodies were found to recognize xBMP-4 specifically and not to cross-react with either xBMP-2 or xBMP-7 which are similar to xBMP-4. In addition, the antibodies recognized dimeric xBMP-4 whereas our previous antibodies recognized the reduced form only. The present antibodies detected an immunoreactive 27 kDa protein in extracts of developing Xenopus embryos from oocyte to tailbud embryo. The xBMP-4 peptide appeared to be monomeric in structure because the molecular weight did not shift upon reduction of disulfide bond(s).