The relationship between the morphological changes in allografted lungs and the cytotoxicity of recipient peripheral blood lymphocytes was studied. Left lung allotransplantations were performed in 14 adult mongrel dogs. Four dogs were treated with cyclosporin A (20 mg/kg/day po.) and four were treated with FK-506 (0.1 mg/kg/day im.) for 10 days. Six dogs were not given any immunosuppressive drugs. Cytotoxicity was measured with the 51Cr release assay in which the donor skin fibroblasts were used as target cells. The histological changes of the grafted lungs were classified into three grades: grade 0; no abnormal findings, grade I; perivascular and peribronchial mononuclear cell infiltration, and grade II; edema, hemorrhage and a fibrin-like substance in the alveolar space in addition to the findings of grade I. The grafted lungs of the dogs with elevated cytotoxicity showed the histological findings of grade II (8 of the 14 dogs) and significantly higher cytotoxicity (44.1 +/- 25.7 per cent) were seen in these dogs than in those with grade 0 (2.0 +/- 3.9 per cent) or grade I (2.6 +/- 1.9 per cent). It is therefore considered that the cytotoxicity against donor skin fibroblasts in recipient peripheral blood lymphocytes is associated with morphological rejection phases in allografted lungs.