Nude mice were transplanted with varying amounts of cultured thymic fragments equivalent to tissue derived from less than 1, 1 or 3 thymuses. All amounts of tissue preserved life, restored skin graft rejection, thymus-dependent allotolerance, anti-sheep erythrocyte antibody responses and serum IgG1 levels to near normal values. Serum IgA levels, IgE and hemagglutinating responses to ovalbumin as well as mitogen proliferation were shown to be more dependent upon the mass of thymus transplanted. In some cases, allogeneic CTF transplantation results showed a greater mass effect, but this was apparent only when less than 1 thymus equivalent was used. These data correlate well with the heterogeneity of the DiGeorge syndrome and variable results seen with human CTF transplantation.