Ever since its discovery, the identification of the specialized conducting system of the heart has been a matter of debate. In some species, a main distinguishing feature under the electron microscope, as compared with ordinary myocytes, is the presence of large pools of juxtanuclear filaments, so called intermediate or skeletin filaments. In the present study, we have adopted the indirect immunofluorescence method and anti-skeletin antisera for the identification of the ventricular conducting system in several species. It was found that anti-skeletin reactivity generally exceeded that of ordinary myocytes. The degree of immunofluorescence could be related to a previous classification model of the differentiation of the conducting cells. It is suggested that skeletin is highly conserved throughout phylogeny and that anti-skeletin may serve as an additional tool for the identification of conducting cells at the light microscopic level.