Successful immunization is described of albino rats with Trypanosoma equiperdum vaccines prepared by treatment of the trypanosomes with para-benzoquinone (or neoarsphenamine). The serum of the immunized rats agglutinated the strain of trypanosomes with which it was prepared and dissolved them if complement was present. However, the resistance against experimental infections was not absolute. A strict parallelism was observed between the agglutinating and sensitizing capacity of the immune sera, suggesting that the same immune body is capable of both of these functions. Observations are also reported concerning the rather remarkable variability of trypanosome strains depending on the history of their passages.