The agent or agents of serum jaundice and of experimental hepatitis are both filtrable and both relatively resistant to heat. A prominent difference between serum jaundice and infectious hepatitis, yet to be explained, lies in the length of the incubation period, which may be quite variable, but in our experimental cases of infectious hepatitis it tended to be from eighteen to thirty days, in contrast to the sixty to one hundred and twenty day incubation period of experimental cases of serum jaundice. Three patients who had recovered from serum jaundice six months before were not found to be immune to experimentally induced attacks of infectious hepatitis. However, little is known regarding immunity in infectious hepatitis, and the ease with which these 3 patients were "reinfected" is not regarded as an indication that the two diseases are fundamentally different. The virus of infectious hepatitis is in the blood and in the