Seventy-seven adult mongrel dogs underwent left lung transplantation to study the immunological detection of posttransplant rejection response using the lymphocyte blastogenesis assay. Experimental animals were divied into 3 groups; control autotransplantation (4 dogs), control allotransplantation (32 dogs) and immunosuppressed allotransplantation (41 dogs). The lymphocyte blastogenesis assay included mixed lymphocyte reaction (MLR), lymphocyte reactivity to lectins (PHA, ConA and PWM) and spontaneous blastogenesis in peripheral lymphocyte culture methods. The latter two tests were performed in serial blood samples up to the 38th posttransplant day. In the control autotransplant group there were no particular changes in posttransplant PHA reactivities. In the animals with lung allograft, results of preoperative MLR between donor and recipient showed no particular relationships to posttransplant rejection response. The recipient, however, showed a decreased response to PHA and an increased spontaneous blastogenesis at the period undergoing rejection which were confirmed by blood samples taken within 2 days before sacrifice of animals whose transplant specimens showed apparent histologic findings of rejection. In the immunosuppressed allotransplantation group, those findings of PHA response and spontaneous blastogenesis had often preceded the rejection episode detected on chest x-ray films, whereas there were no particular changes in both parameters of dogs bearing enhanced allografts. The serial detection of spontaneous blastogenesis and PHA reactivity in peripheral lymphocyte may be one of the useful methods for early prediction of rejection episode in lung transplantation.