Activins are involved in many physiological and pathological processes and, like other members of the transforming growth factor-beta superfamily, signal via type II and I receptor serine kinases. Ligand residues involved in type II receptor binding are located in the two anti-parallel beta strands of the TGF-beta proteins, also known as the fingers. Activin-A mutants able to bind ActRII but unable to bind the activin type I receptor ALK4 define ligand residues involved in ALK4 binding and could potentially act as antagonists. Therefore, a series of FLAG-tagged activin-A/C chimeras were constructed, in each of which eight residues in the wrist loop and helix region (A/C 46-53, 54-61, 62-69, and 70-78) were replaced. Additionally, a chimera was generated in which the entire wrist region (A/C 46-78) was changed from activin-A to activin-C. The chimeras were assessed for ActRII binding, activin bioactivity, as well as antagonistic properties. All five chimeras retained high affinity for mouse ActRII. Of these, only A/C 46-78 was devoid of significant activin bioactivity in an A3 Lux reporter assay in 293T cells at concentrations up to 40 nM. A/C 46-53, 54-61, 62-69, and 70-78 showed activity comparable with wild type activin-A. When tested for the ability to antagonize ligands that signal via activin type II receptors, such as activin-A and myostatin, only the A/C 46-78 chimera showed antagonism (IC(50), 1-10 nM). Additionally, A/C 46-78 decreased follicle-stimulating hormone release from the LbetaT2 cell line and rat anterior pituitary cells in primary culture in a concentration-dependent manner. These data indicate that activin residues in the wrist are involved in ALK4-mediated signaling. The activin antagonist A/C 46-78 may be useful for the study and modulation of activin-dependent processes.