Reports results of clinical, morphologic, and laboratory studies carried out in 18 patients allografted with spleen fragments into the greater omentum following splenectomy for a traumatic injury of the spleen, and in 20 splenectomized dogs allografted with spleen fragments or a spleen cell suspension into various parts of the abdominal cavity. Spleen tissue regeneration in the allografts was found to be nearing completion 6 to 8 weeks after the transplantation. The regeneration took less time to be completed in dogs grafted with spleen fragments than in those grafted with a cell suspension, but occurred at similar rates whatever the site in the abdominal cavity into which the fragments or suspension had been grafted. In the allografted patients, the incidence of posttransplantation suppurations was lower than in a similar group of patients who had undergone splenectomy without allografting.