Purified CD4+ lymph node T cells were sorted into two populations on the basis of their expression of CD45RB (CD45RBhi and CD45RBlo) and injected into congenic severe combined immunodeficient (SCID) mice. After a period of time that was dependent on the number of cells injected, the SCID mice that received CD45RBhi/CD4+ T cells developed a wasting disease that was not seen in SCID mice that received the CD4+/CD45RBlo cells or whole lymph node cells. At death, SCID mice that received the CD4+/CD45RBhi cells had increased spleen and lymph node cellularity compared with normal SCID mice and SCID mice that received the CD4+/CD45RBlo T cells. The spleen and lymph node contained CD4+ cells and neither CD8+ nor surface immunoglobulin M-positive cells, plus a population of cells that did not express any of those markers. At necropsy, the SCID mice that received the CD4+/CD45RBhi cells had significant hyperplasia of the intestinal mucosa with significant lymphoid cell accumulation in the lamina propria. Interestingly, mice that received mixtures of whole lymph node or purified CD4+ cells with CD4+/CD45RBhi cells did not develop weight loss, indicating that the unseparated CD4+ population contained cells that were capable of regulating the reactivity of the CD4+/CD45RBhi cells.