Platelet satellitosis resulted in an elevated high-peroxidase-activity value (9.9% versus normal range 0 to 3.65%) of the automated leukocyte differential count performed by the Technicon Hemalog D. Platelet satellitism occurred in Wright-stained smears of EDTA-anticoagulated blood, as well as in the effluent of the peroxidase channel of the Hemalog D. All platelets took up the perosidase stain. The rosette-like clusters of platelets and neutrophils were interpreted as single, large, intensely stained leukocytes resulting in the elevated high-peroxidase-activity value.