The restoration of immune functions was followed in dogs for 101 days after fractionated total body irradiation and autologous transfusion of peripheral blood leukocytes (PBL) or bone marrow (BM) cells. Median numbers of 0.9 X 10(5) granulocyte-macrophage progenitor cells per kilogram of body weight were transferred in either group of recipients. The following parameters recovered more rapidly in PBL recipients as opposed to BM recipients: total blood lymphocyte, T- and B-cell counts, serum levels of immunoglobulins IgM and IgA, in vitro blastogenic responses after stimulation with concanavalin A and pokeweed mitogen, and in vitro plasma cell formation after polyclonal B-cell activation with pokeweed mitogen with or without lipopolysaccharide. No major differences were noted for the restoration of serum IgG levels. Circulating lymphocyte and T-cell numbers remained subnormal for more than three months in both groups, whereas B-cell numbers and serum levels of IgA continued to be depressed in BM recipients only. Thus, autologous PBL restored immune functions more rapidly than did BM. Transplantation of PBL, alone or in addition to autologous BM, might also shorten the period of immunodeficiency after cytoreduction in a variety of malignancies in man.