Between one and six cultured human fetal pancreata were allografted into five insulin-dependent diabetic recipients and their progress monitored for a year on each occasion. To prevent rejection the tissue was cultured for 1-3 weeks before transplantation, the HLA-DR antigens of the donor tissue were matched with those of the recipient when a single pancreas was used, and four of the recipients were immunosuppressed, three because of coexisting renal grafts. Graft function was observed transiently in one of the recipients. In three others human fetal pancreas was recovered 9-14 months after transplantation, although it was being slowly rejected during this time. Beta cells were present in the graft but did not function adequately to enable immunoreactive C-peptide to be measured in peripheral blood. The issues of rejection and immaturity of the human fetal pancreas will need to be surmounted if the potential of the human fetal pancreas to normalize blood glucose levels in diabetic man is ever to be realized.