1. A simple test for the detection of leukoagglutinating properties of serum is described.
2. A thermolabile serum system which inhibits the leukoagglutination was observed.
3. Sera of about two thousand normal subjects and six hundred patients with various diseases were tested. The sera of nineteen of the patients gave positive reactions.
4. The serologic and physicochemical properties of the leukoagglutinating substance were studied, and evidence is presented in favor of its being an antibody.
5. The role of the leukoagglutinating substance in the development of leukopenia is discussed, as well as the probable type of antibody responsible for the leukoagglutination reaction.