Canines were studied to determine the efficacy of peripheral blood collection by semicontinuous centrifugation to procure sufficient numbers of hematopoietic cells for marrow reconstitution. CFU-C assays on fresh and cryopreserved peripheral blood cells were compared to bone marrow aspirates. Attempts were made to partially separate hematopoietic cells from immunocompetent cells during buffy coat collection and by incubation with activated cyclophosphamide. Yields from 4 runs of semicontinuous centrifugation averaged 1.7 x 10(5) CFU-C compared to 1.9 x 10(5) CFU-C for standard marrow aspiration. Ratios of approximately 5/1 marrow to peripheral blood mononuclear cells (MNC) were found. Autologous transplantation with 1.6 x 10(4) CFU-C per kg resulted in evidence of marrow reconstitution within two weeks following otherwise lethal chemotherapy. Recovery of CFU-C following cryo-preservation was 82% for marrow and 78% for peripheral blood. Selective depression of MLC activity occurred when peripheral blood MNC were incubated for 30 minutes with activated cyclophosphamide. MLC was inhibited by greater than 90% while 70% of CFU-Cs were retained. It was concluded that peripheral blood may be a practical alternative to marrow for transplantation studies.