To determine whether survival of allografted rat parathyroid tissue could be enhanced either by the administration of antidonor serum (enhancing serum) or by tissue culture of the graft, parathyroids from Lewis x Brown Norway F1 (LBNF1) rats were transplanted beneath the fascia and between hamstring muscle fibers of hypocalcemic parathyroidectomized Lewis recipients (serum calcium 4.3 +/- 0.13mg%--mean +/- SEM). A drop in serum calcium below 7.5 mg% indicated rejection. Seven group 1 rats that received isogeneic parathyroids recovered from their hypocalcemic state and remained normocalcemic indefinitely. Seven group 2 rats that received LBNF(1) parathyroids experienced a reduced mean maximal response (MMR) of calcium increase of 4.1 +/- 0.2 mg% and the grafts had a median survival time (MST) of 16.0 +/- 1.32 days. Group 3 rats that received 1 ml of enhancing serum by intraperiotoneal injection, cytotoxic titer 1:128, on alternate days for five doses experienced an MMR of 5.0 +/- 0.7 mg% and had an MST of 19.3 +/- 1.07 days. When enhancing serum, cytotoxic titer 1:256, was administered to group 4, the MST of the grafts was increased to 29.7 +/- 1.19 days. When LBNF(1) parathyroid tissue was preserved in tissue culture for 14 to 21 days and then transplanted into Lewis recipients, the MST was 22.5 +/- 1.48 days. The administration of enhancing serum to recipients of cultured parathyroids further extended graft survival, MST 57.5 +/- 2.32 days. In summary, survival of allografted parathyroid tissue can be prolonged by the administration of enchancing serum or tissue culture of the graft. When enhancing serum was administered to recipients of cultured allografted parathyroid tissue, survival was further increased.