SUMMARYThe behavior of BALB/c myeloma transplants in (C57BL/ 10 BALB/c) F1hybrids was investigated. The F1hybrids were less susceptible to the grafts than were the parental BALB/c hosts. Host susceptibility was X-ray and cyclophosphamide sensitive, but an immune hypothesis was not favored since no memory was demonstrable. Males were less susceptible than females, and rabbit antithymus serum had no effect on susceptibility.