Polyclonal antibodies capable of reacting with amphibian bone morphogenetic protein (BMP-2 and -4) were raised in rabbits by immunization with a synthetic 21 amino acid peptide which corresponds to a sequence residing in the mature protein of Xenopus BMP-2 (xBMP-2). The antibodies recognized an embryonic BMP as well as mammalian and bacteria expressed recombinant xBMPs. The antibodies detected, under reducing conditions, a 30 kDa protein in the extract of oocytes and embryos during early development. Interestingly, acidification of the extract from each developmental stage yielded a protein band of smaller molecular weight of 18 kDa, which is similar in size to reduced form of mature BMPs purified from mammalian species. Two-dimensional electrophoresis employed to examine the molecular weight of unreduced forms using the antibody, revealed that both molecular forms are monomeric in the embryos. The result suggests that at least BMP-2 mRNA previously detected in early embryos, is translated into peptide but the dimerization may be incomplete or strictly limited in these embryos.