We have undertaken a series of experiments using a canine model to determine the minimal number of cryopreserved autologous bone marrow or peripheral blood mononuclear cells needed to protect animals from otherwise lethal total body irradiation. We further have compared the kinetics of engraftment using hemopoietic cells from these two sources. Animals engrafted with bone marrow required .25 X 10(8) nucleated bone marrow cells/kg to protect them; those engrafted with peripheral blood mononuclear cells required 6.0 X 10(8) cells/kg. Myeloid and platelet recovery appeared more rapid after bone marrow infusion while lymphoid recovery was more rapid when peripheral blood mononuclear cells were used.