An antibody against the Xenopus activin/inhibin beta(B) subunit (94-107) was raised in a rabbit. Using this antibody, the distribution of activin/inhibin beta(B) immunoreactivity in the pituitary of adult X. laevis was studied. Beta(B) immunoreactivity was detected in gonadotrophs, thyrotrophs, and somatotrophs under light microscopy. Electron microscopy revealed that a beta(B)-immunoreactive substance exists in LH, TSH, and GH granules, in contrast to findings in the rat and goldfish. These results indicate that the expression of activin/inhibin beta(B) in pituitary cells is not consistent among vertebrate species.