The majority of the experience in the US and worldwide has been with horse ATG (h-ATG) plus CsA as initial therapy in SAA. Rabbit ATG (r-ATG) plus CsA has been employed successfully in about 1/3 of cases in those who are refractory to initial h-ATG/CsA (current NHLBI Protocol 03-H-0249). In recent years, h-ATG and r-ATG have been used interchangeably in treatment-naive patients, and initial therapy with r-ATG/CsA is now frequent in the US and the only option in Europe and Japan, where h-ATG is no longer available. An active NHLBI randomized study is comparing the efficacy of h- and r-ATG as initial therapy in SAA, and the results from a recently completed interim analysis suggest that the hematologic response rate ultimately may not be comparable between these two agents (Protocol 06-H-0034). There is no published report on the outcome of repeat immunosuppressive therapy in those patients refractory to initial r-ATG/CsA, and thus the management of these patients is uncertain. We therefore propose this study of h-ATG/CsA in SAA patients who are refractory or have a suboptimal response to r-ATG.