A highly sensitive method for protein visualization following electrophoresis and protein blotting was developed. The method is based on the light-emitting reaction of luminol and hydrogen peroxide catalyzed by horseradish peroxidase. The luminescent assay can be applied either to the native gel or after protein blotting, and it has a sensitivity two orders of magnitude higher than that achieved with chromogenic detection systems. Analogous to autoradiography the luminescent signal is recorded on an X-ray film with similar sensitivity. We present several examples of application emphasizing the general versatility of this innovative method.