This report gives the results of 412 experiments testing the prophylactic and therapeutic value of 190 chemical compounds on poliomyelitis and St. Louis encephalitis infections in white Swiss mice. This study was initiated in 1939 and is part of a long range effort to test systematically the usefulness of a wide variety of compounds, both old and new, in preventing or in modifying the outcome of certain experimental virus infections. In view of our special interest in poliomyelitis, two neurotropic viruses were selected to serve as test experimental infections. One was the mouse-adapted Armstrong Lansing strain of poliomyelitis, and the other was the Webster strain of mouse-adapted St. Louis encephalitis virus. The chemical compounds were supplied to us by a cooperating group of manufacturing concerns.2 Among the 190 compounds tested we found none which possessed any significant prophylactic or therapeutic action on the two test experimental infections.