A reliable method for the production of ACTH-antibodies with high titres is described. ACTH was conjugated by means of ethyl-carbodiimide. Guinea pigs and rabbits were immunized. The sensitivity of the resulting antisera was evaluated by plotting a standard-curve with each antiserum. With the best antisera 1 pg ACTH/ml could be measured. Specificity of the antisera was tested by setting up antiserum dilution curves with labelled αp1-39 ACTH, βh1-39 ACTH and β1-24 ACTH. Surprisingly, most of the antisera appeared to react specifically with the N-terminal portion of the ACTH-chain. 1 Supported by Deutsche Forschungsgemeinschaft, Bad Godesberg (SFB 87/Proj. 18).