Using donor lung antigens as targets, two test methods, a rosette-forming cell test (RFC) and indirect immunofluorescence test (indir. IF), were applied to focus on the first set lung allograft rejection in canine recipients with and without immunosuppressive treatment. In both groups the first sign of humoral or cellular sensitization became evident on the 4th postoperative day, which might answer some unsolved problems in early lung graft damage, recently discussed as a manifestation of certain immunologic reactions. The tests used, indir. IF and RFC, revealed a steadily climbing tendency of immunization in untreated recipients and more variable and reversible positive results in the immunosuppressively treated group. No correlation between cellular and humoral immunity could be proven by our tests. The fact that we found no incorrect positive results in RFC suggests that this test provides important hints for the diagnosis of lung allograft rejection.