Hemophilus pleuropneumoniae is highly pathogenic for swine when given intranasally. As few as 100 organisms induce characteristic porcine contagious pleuropneumonia (PCP) and, when as many as one-half million are given, the infection usually proceeds to a fatal termination. While the organism is highly pathogenic when introduced by way of the respiratory tract, it is innocuous when given subcutaneously even in large numbers. Swine that have been inoculated subcutaneously are rendered solidly immune to infection with H. pleuropneumoniae intranasally. The marked pathogenicity of H. pleuropneumoniae for swine has been contrasted with the lack of pathogenicity of another swine Hemophilus, H. influenzae suis. It has been pointed out that, in its high degree of pathogenicity, in its pneumotropism, and in its immunogenicity by a non-respiratory route of inoculation, H. pleuropneumoniae appears to resemble certain viruses more than it does a bacterium.