Eosinophil peroxidase and myeloperoxidase play an important role in the host defense. Both enzymes are present in bone marrow, synthesized by blood progenitor cells. This research investigated the kinetic properties of peroxidases under immunostimulation in guinea pig bone marrow. Results suggest that there are at least two myeloperoxidase isozymes and at least three eosinophil peroxidase isozymes in guinea pig bone marrow and that some of these isozymes are expressed upon immunostimulation.