A series of 22 outbred dogs has been subjected to therapy with BW 57–322 in conjunction with pulmonary homografting to achieve suppression of the immunological reaction. Of the 22 dogs, 4 were unaffected by the therapy, 10 were affected as manifest by an attenuation of the homograft response and by qualitative histological changes, and 5 had no morphological evidence of a homograft rejection. One of these dogs is surviving with a structurally normal lung at 238 days post-grafting. Certain unique histological lesions in the grafts undergoing an attenuated homograft response and in the host's own lung are described.