Sixty-five mongrel dogs were subjected to left lung allotransplantation with immunosuppression. Serial assessment of graft function and rejection was made by bronchospirometry, xenon Xe 133 and radioalbumin scintiscans, and open lung biopsy. Correlation of data shows (1) scintiscans should be performed at not more than 48-hour intervals during the first 21 days; (2) early rejection is suggested by a decrease in perfusion in the presence of a satisfactory topographical ventilation scan; and (3) late rejection is indicated by concomitant deterioration of perfusion and ventilation scans, but cannot be distinguished from progressive infection by scintiscan techniques.