Neonatal spleens were grafted under the kidney capsule of adult syngeneic mice which were either normal, thymectomized, splenectomized or both thymectomized and splenectomized. After 14 days in situ, grafts were exised and the total cell number, the number of Thy-1 and Ig-positive cells, the plaque-forming cell (PFC) response after in vivo immunization with SRBC, and the suppressive activity in vitro on immune cells were determined. The expansion of the T-cell-precursor pool was not dependent upon the presence of the host thymus, nor was the antibody response, which was of neonatal type, i.e. with low PFC response and high suppressive activity. Host splenectomy enhances dramatically the proliferation of neonatal spleen graft cells and their ability to respond to SRBC. This enhancement is essentially due to a host cellular contribution, and is not observed when the graft is an adult spleen fragment. These results suggest that the spleen itself could have a regulatory role in postnatal lymphoid development.