This dissertation is concerned mainly with the occurrence of the Forssman heterophilic antigen in certain bacteria. The first chapter deals with the lipoid fraction or hapten of the Forssman antigen and with the power of bacterial suspensions to convert this hapten into a complete antigen. In the second chapter are described the results of efforts to demonstrate Forssman's antigen in certain bacteria by immunization of rabbits and testing the serum for Forssman antibody. The third and last chapter gives the results of a series of observations on the presence of Forssman antibody in the serum of 219 patients suffering from infectious diseases. The book will interest those who are concerned with heterophilic antigens and antibodies.