The mechanisms by which rapamycin (RAPA) and/or cyclosporine induce unresponsiveness to allografts were investigated in a rat model. Buffalo (BUF, RT-1b) heart allografts were rejected by Wistar-Furth (WFu, RT-1u) recipients at a mean survival time (MST) of 6.5 +/- 0.5 days. A 14-day course of RAPA (0.8 mg/kg) delivered intravenously by an osmotic pump prolonged BUF allograft survival to 76.1 +/- 23.4 days (P < 0.001). Adoptive transfer of 30-50 x 10(6) spleen and lymph node T cells that had been isolated on day 40 postgrafting from CsA- or RAPA/CsA-treated hosts into lightly irradiated (6 Gray) secondary WFu recipients prolonged BUF graft survival from 9.8 +/- 1.2 to 29.2 +/- 11.0 (P < 0.01) and 58.2 +/- 38.9 days (P < 0.004), respectively. T cells transferred from animals treated with RAPA alone failed to prolong graft survival. In contrast, sera isolated on day 40 postgrafting from WFu primary hosts treated with RAPA alone or with the RAPA/CsA combination, but not with CsA alone, extended the survival of BUF hearts: 3 ml serum from RAPA-treated hosts prolonged BUF heart survival to 76.6 +/- 31.3 days (P < 0.002) and from RAPA/CsA-treated hosts to 47.1 +/- 12.8 days (P < 0.001). The effect of serum was immunologically specific: it did not prolong the survival of third-party outbred Sprague Dawley heart allografts. Although the IgM fraction (0.2 mg) purified from the serum of RAPA-treated recipients was ineffective (10.6 +/- 0.8 days; NS), an equal amount of the IgG fraction significantly (P < 0.002) prolonged BUF heart allograft survival to 26 days (n = 4). Thus, hosts treated with RAPA or a RAPA/CsA combination develop IgG antibodies that mediate the unresponsive state toward allogeneic heart allografts.