Summary. Dogs affected with cyclic haematopoiesis (CH) and experiencing recurrent 12-day cycles of neutropenia were supralethally irradiated and transplanted with marrow from their normal littermates. Following transplantation of the normal marrow, dogs with CH experienced no further cycles of neutropenia. Platelet and reticulocyte numbers also ceased to fluctuate cyclically in these dogs. The marrows of dogs with CH were transplanted into unaffected supralethally irradiated littermates. Prompt engraftment occurred in the recipient dogs with subsequent cyclic rises and falls of the numbers of platelets, reticulocytes and neutrophils. In the post-transplant period, immunosuppressant drugs were not used in three of the recipients and the fourth received only a single 5 mg dose of methotrexate.