1. Rhesus monkeys immunized repeatedly with homologous skin-grafts and/or blood given i.v., produced potent leukoagglutinating isoantibodies. The antisera are directed against more than one anti-genic determinant, though immunization was done with skin or blood from one donor only. 2. Sera from monkeys with a history of one or more pregnancies sometimes contain weak leukoagglutinating antibodies. If subsequently immunized with homologous blood, strong leukoagglutinins are rapidly produced. 3. Sera from monkeys without a history of pregnancy or experimental isoimmunization do not contain leukoagglutinating or hemagglutinating antibodies. 4. Hemagglutinins were demonstrable in a few cases after immunization, but only if blood transfusions were included in the immunization schedule. 5. Characteristics of the leukoagglutinating antibodies and their use for the determination of the “antigenic profile” of leukocytes are presented. The limitations of typing for leukocyte compatibility with the available, not monospecific, isoantisera are emphasized.