Activins are members of the TGF-I? super-family. There are 4 mammalian activin subunits (I?A, I?B, I?C and I?E) that combine to form functional proteins. The role of activin A (I?AI?A) is well characterized and known to be a potent growth and differentiation factor. Two of the activin subunits (I?C and I?E) were discovered more recently and little is known about their biological functions. In this review the evidence that activin-I?C is a significant regulator of activin A bioactivity is presented and discussed. It is concluded that activin-I?C, like other antagonists of activin A, is an important growth regulator in prostate health and disease.