Summary Diffusion chambers were filled with bone marrow from 2–4 weeks old rabbits and dogs, and implanted into the peritoneal cavity of adult animals for 28–45 days. The diffusion chambers were encapsulated by omentum in both dogs and rabbits, and the character of the omental response was influenced by the chamber contents. Empty chambers were enveloped by omentum composed principally of fibrous-fatty tissues, with a thin fibrous layer adherent to the chamber walls. Chambers containing allogeneic marrow evoked a marked cellular and angiogenic response in the adherent omentum. In rabbits, lymphocytes were the predominant cell type. In dogs, large ecchymotic haemorrhages were found immediately adjacent to the chambers, with aggregations of lymphocytes in the surrounding dense fibrous tissue. Diffusion chambers containing xenogeneic marrow were surrounded by a thick fibrous tissue capsule, without any adhesion of the omental tissues to the chambers. The internal layer of the omental capsule was necrotic and chambers were bathed in sero-sanguinous fluid. It was concluded that antigens on fragments of marrow cells escaping from the diffusion chambers had evoked the various omental responses which were observed.