Neonatally thymectomized C3Hf/Bi mice could be restored immunologically (skin allograft rejection, graft-versus-host capacity, and delayed hypersensitivity to sheep red cells) by implantation of cell-impermeable diffusion chambers (0.22 or 0.10 μ average pore size filters) containing syngeneic or allogeneic nonlymphoid functional thymomas or normal thymus from 20-day-old donors. Three functional thymomas were equally effective in restoring the thymectomized hosts. The treatment was performed at 20 days of age and the chambers were implanted intraperitoneally. Ten other tumors of thymic and nonthymic origin could not restore the thymectomized hosts when implanted in diffusion chambers. A variety of normal tissues (spleen, lymph nodes, Peyer's patches, cecal appendix, submaxillary gland, liver, ovary, adrenal, thyroid, or hypophysis) as well as empty diffusion chambers could not induce any immunological restoration when implanted within cell-impermeable chambers into neonatally thymectomized mice. Diffusion chambers made with plastic rings and cemented filters of 0.45 or 0.30 μ average pore filter allowed passage of significant numbers of cells both within and outside the chambers. The chambers prepared with the 0.22 or 0.10 μ pore size did not permit cell traffic.