Human total lung homotransplantation requires either a source of immediate postmortem lungs, adequate storage techniques, or the sacrifice of an entire lung by a living donor. Necessitating sacrifice of smaller amounts of normal lung tissue by the donor would appear more reasonable and still be of significant benefit to the recipient. The anatomic relations of the human and canine left lower lobe make it suitable for homotransplantation, and in addition it represents 25% of the total lung function. At the same time, as an experimental technique it provides us with a better model to study homograft response of transplanted lung tissue than does total lung homotransplantation since it has a less associated mortality and requires little or no blood for transfusion at operation. Thus pulmonary alterations due to incompatible blood were avoided.1,2 This work was planned to study the functional and morphological changes associated with (1) homotransplantation of the