(1) Among 3,100 persons immunized against yellow fever with virus and immune serum over a period of five years, 89 cases of jaundice have been traced.(2) The symptoms are those of a hepatitis and closely resemble those produced by common infective hepatic jaundice, cases of which have frequently been noted as occurring in the same areas.(3) The average period between the time of inoculation and the development of hepatitis is between two and three months.(4) Attention is directed to the occurrence of hepatitis in horses, usually two to three months after immunization against the viruses of horse sickness and equine encephalomyelitis, and also after the injection of horse serum containing antitoxins against Cl. welchii toxins. Similar symptoms were observed, though to a lesser extent, in normal horses.(5) The only factor common to the inoculated horses and men was the injection of homologous proteins, either in sera or in tissue extracts.(6) The only theories which at present explain the observed facts are that either (1) a hepatotoxic virus is introduced with the virus inoculum or that (2) two factors combine to induce the hepatitis (a) a hepatotoxic substance present in the homologous sera or tissue extract injected and (b) an infective agent which, in the case at least of human beings, is probably the causal agent of common infective hepatic jaundice.