Abstract 1. 1. The bioassay dose-response relationship is described for the nerve growth factor (NGF). A simple method for determination of the Biological Unit (BU) is described. 2. 2. It is proposed that the potency of any nerve growth promoting protein be expressed in terms of specific activity (BU/mg). If BU is defined in terms of the amount of protein which elicits a 3 + response, the specific activity is a dimensionless number.