Modulation of activin and other TGFβ superfamily signaling is the primary mechanism of action for both follistatin (FS) and FS-like 3 (FSTL-3). However, most studies of these ligands use activin A due to its wide availability. We have now tested the ability of FS288 and FSTL-3 to bind and neutralize activin B relative to activin A. Activin B bound to both FS and FSTL-3 at a potency approximately 10-fold lower than that of activin A. Moreover, whereas both activins had similar biological activity in 293 cell reporter assays, FS and FSTL-3 were approximately 3-fold more effective in neutralizing activin A relative to activin B. These results suggest that neutralization of activins A and B by FS and FSTL-3 are not identical, so that the relative activity of each activin in tissues where both are produced, such as in the ovary, could be quite different. In addition, biological systems that use primarily activin B, but which have been examined in vitro using activin A, may need to be reevaluated to determine the actual physiologic roles of FS or FSTL-3.