Mesoderm induction requires interaction between cells of the animal an
d vegetal hemispheres of the embryo. Several molecules have been propo
sed as candidates for mesoderm-inducing signals, with activin a partic
ularly strong candidate. However, it has not been possible to inhibit
mesoderm formation in vivo by specifically blocking activin action. Fo
llistatin is able to inhibit the action of activin but not that of the
mature region of Vg1, a member of the transforming growth factor beta
family. Follistatin therefore provides a useful tool for distinguishi
ng between signalling by these two factors. We have overexpressed Xeno
pus follistatin mRNA and analysed the expression of several mesodermal
markers. Our results show an inhibition of mesodermal formation by fo
llistatin in a concentration-dependent manner, showing the requirement
of activin for mesodermal induction.