The studies are designed to obtain information regarding the pathogenesis of human canine cyclic hematopoiesis (CH). Residing in the Knoxville-Maryville area is a family of four generations in whom cyclic neutropenia is or was present in four members. One other member has a persistant leukopenia without cyclic behavior. We found, as other investigators have determined, that in the cyclic condition the monocytes are out of phase with the granulocytes. Located at our institution is a colony of grey collie dogs which demonstrate cyclic hematopoiesis. Growth of canine marrow in millipore diffusion chambers has been carried out. The results indicate that marrow from dogs with CH has a cytopoietic activity that equals or exceeds that of normal dogs, suggesting that the grey collies do not have a restricted stem cell pool. The titers of the cyclic nucleotide, cAMP, were determined in the serum and urine obtained from CH dogs each day throughout their cycle. Definite cyclic fluctuations were found in the urine, a condition which was not present in normal dogs. During the next grant period, kinetic studies will be carried out on peripheral blood and bone marrow of both the patients and the dogs with cyclic disorders.