We investigated the correlation between the donor specific CTL activities and pathological findings of the grafted lungs by using mononuclear cells obtained from BAL fluids (BAL), peripheral blood (PBL), spleen (SpL) and grafted lungs (GIL) in canine lung allotransplantation. Twenty eight mongrel dogs underwent left lung allotransplantation. Fifteen dogs were not given immunosuppressive drugs. Thirteen dogs were given cyclosporine a 20 mg/kg/day per os for ten days postoperatively. Dogs were monitored by chest roentgenograms and cytotoxic activity tests of which targets were donor's fibroblasts. Pathological findings were classified by Prop's classification. The results were as follows. In latent and vascular phases no elevations of cytotoxic activities were demonstrated in PBL, BAL and GIL. But in early alveolar phase the cytotoxic activity was elevated to 19.83 +/- 14.54% in BAL. In late alveolar phase the highest cytotoxicities of 30.83 +/- 15.28% in PBL, 72.63 +/- 7.36% in BAL and 70.07 +/- 6.34% in GIL at E:T = 100:1 were demonstrated. In pneumonia cytotoxicity did not elevate at all in any effectors. As a result of them, it clearly shows that BAL is a good tool for the diagnosis of lung rejection.