Natural suppressor (NS) cell lines were derived from the spleen and thymus of adult mice using procedures previously used to derive NS cells from neonatal spleen. Adult spleen-derived NS cells showed slightly greater suppression of the MLR than neonatal spleen-derived NS cells, and thymus-derived NS cells showed the least suppression. Adult spleen-derived NS cells prevented death from lethal GVHD when administered to suble-thally irradiated weanling mice that received an otherwise lethal GVHD provoking inoculum. The stable cell surface phenotype of adult tissue-derived NS cells was similar to that previously described for neonatal and TLI spleen-derived NS cells: strongly positive for Thy 1.2, Ly-5, and asialo-GM1 antigens while negative for Ly-1, Ly-2, L3T4, MAC-1, and surface immunoglobulin.