Canine cyclic hematopoiesis (CH) is an autosomal recessive disease of gray collie dogs that is characterized by 14-day cycles of neutropenia, monocytosis, thrombocytosis, and reticulocytosis. Platelets from CH dogs have decreased dense-granule serotonin pools and decreased aggregation responses to collagen, platelet-activating factor (PAF), and thrombin. Recombinant granulocyte colony-stimulating factor (rG-CSF) was administered (5 micrograms/kg, b.i.d.) to four CH and six normal dogs to determine if G-CSF therapy corrected qualitative platelet defects in CH dogs. Neutrophil counts increase to greater than 25,000 cells/microliters within 24 h after starting treatment in all dogs. Treatment with G-CSF blocked neutropenic episodes in the CH dogs. Platelet aggregation, and serotonin content and secretion were significantly (p less than 0.05) decreased in the CH dogs both before and during recombinant human (rh) G-CSF treatment compared to normal dogs. Neutrophil myeloperoxidase, a primary granule enzyme, was significantly (p less than 0.05) decreased in CH dogs and was not corrected by rhG-CSF treatment. Administration of rG-CSF to CH dogs eliminated cell cycles but apparently did not correct cellular defects in CH dogs. Identification of primary biochemical defects in cells from CH dogs may be crucial to investigating the biochemical basis for cyclic hematopoiesis.