Disclosed is a method for identifying an agent that increases red blood cell levels by contacting an agent with ActRII； adding activin； quantifying the agents efficacy at inhibiting complex formation between ActRII and activin； and evaluating the effect of the agent on red blood cell levels. Further disclosed is the use of an activin-ActRII antagonist in the manufacture of a medicament for treating or preventing anemia, wherein the antagonist is selected from: an antibody that binds to activin and disrupts ActRIIa binding, an antibody that binds to activin and disrupts ActRIIb binding, an anti-activin βA, βB, βC or βE antibody, an anti-ActRIIa antibody, an anti-ActRIIb antibody, follistatin, and inhibin.
