Binds directly to activin and functions as an activin antagonist. Specific inhibitor of the biosynthesis and secretion of pituitary follicle stimulating hormone (FSH). Monomer. Isoform 2/FS-288 interacts with GDF11. N-glycosylated. FS-303 may be derived from mature isoform 1 by post-translational proteolytic cleavage. It is the most abundant form in ovaries (PubMed:8340384).