Abstract: The present invention provides a method for determining the in vitro potency of antibodies, in particular anti-Rhesus D positive antibodies. Such antibodies may e.g. be used in the prophylaxis of hemolytic disease of the newborn (HDN), treatment of idiopathic thrombocytopenic purpura (ITP) and prevention of sensitization to the Rhesus D antigen after mistransfusions of RhD(+) blood to RhD(?) individuals. The invention also provides a method for monitoring the release of label from a RhD(+) red blood cell, a method for determining the ability of an antibody to induce the release of label from a RhD(+) red blood cell, and a method for determining whether a manufactured anti-RhD antibody fulfils a predefined release criterion. The invention further provides a kit for measuring the potency of an antibody.