Adjuvant induced arthritis (AA) in Buffalo rats is a chronic progressive disease that responds very well to treatment with myeloablation and rescue with autologous BM. These previous results were obtained by conditioning with a lethal single dose of TBI. In the present study various other conditioning regimens were compared with single dose TBI. Fractionated TBI using adjusted total dose was equally effective. CY and BU when used as single agents at the highest tolerated dose were less effective. Combinations of CY (2 x 60 mg/kg) with lower dose (4 Gy) TBI and of BU with CY were as beneficial as high-dose TBI. These results indicate that a very intense reduction of T lymphocytes, in the order of 3-4 log, is required for obtaining the highest rate of long-lasting complete remissions. A similar conclusion was reached from our studies of various conditioning regimens in rats suffering from experimental allergic encephalitis (EAE). If extrapolated to the clinic, such a degree of T lymphocyte eradication poses upper limits to the number of T lymphocytes that can be safely reintroduced with the autograft. The exact limits cannot be derived from these experiments because the addition of autologous T lymphocytes to the graft yielded different results in the two models of autoimmune disease (AID).