Heterotransplantation of 1.0 X IO7 AK-4 cells of AKR mice into cheek pouches of normal and cortisone-treated adult Syrian hamsters resulted in measurable but tran sient growth; small vascular tumors which appeared at 5-7 days regressed completely by 10-14 days. Noncortisonized hamsters vigorously rejected second-set grafts, which failed to vascularize, in considerably less time, in either the previously exposed or contralateral pouch. Hamsters treated with cortisone during and following

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... n of the first-set graft also vigorously rejected second-set grafts at 2 weeks following first-set grafting but accepted second-set grafts, in 50 per cent of the instances at 4-6 weeks, in either the previously exposed or contralateral pouch. This acceptance of second-set grafts was in contrast to the failure of hamsters pretreated with cortisone alone to support the growth of first-set grafts. Successful second-set grafts in cortisone-treated hosts grew progressively for 10-14 days, after which interval the grafts necrotized and ulcerated. Despite evidence of locally infiltrating cheek pouch tumors at death at 4-6 weeks, in no instance was dis seminated leukemia observed. Retransplantation of hamster-borne tumors to mice, however, resulted in selective death of AKR mice with disseminated disease, indicating retention of strain-specificity and the capacity of the cells to generalize. The possible relation between heightened susceptibility to AK-4 heterografts in the hamster pouch, and tumor homograft enhancement in the mouse is discussed, as is the significance of this study with respect to the concept of "immunological privilege" of the cheek pouch.