The expression of Ia antigens by lymphocytes in bronchoalveolar lavage (BAL) was evaluated after canine lung allotransplantation with immunosuppression using FK-506. The expression of Ia antigens labeled using an OKIa-1 monoclonal antibody from Ortho Diagnostic Systems was measured by flow cytometry. Twenty-three adult mongrel dogs underwent left lung allotransplantation and were treated with FK-506 at a dose of 0.1 mg/kg/day intramuscularly until death. Allograft rejection was evaluated microscopically. The percentage of OKIa-1-positive cells among the BAL lymphocytes was 34.8% +/- 8.9% (mean +/- SD) from the allografted lungs showing no rejection, whereas it was 68.8% +/- 16.2% from the allografted lungs which showed rejection (P < 0.01). When a diagnosis of rejection was made prior to pathologic examination by an OKIa-1-positive lymphocyte percentage of 42% or more, the sensitivity, specificity, and accuracy of diagnosis were 100%, 87.5%, and 94.1%, respectively. Subsequently, pulse steroid therapy was attempted in those dogs with a high rate of OKIa-1-positive lymphocytes in the BAL. In 3 of 4 dogs showing histological signs of rejection, a decrease in the rate of OKIa-1-positive cells among the BAL lymphocytes corresponded to an improvement in pathologic diagnosis. In two dogs with bacterial pneumonia and pulmonary vein thrombosis, densities indistinguishable from those of rejection were seen on chest roentgenograms, but in these dogs the rate of OKIa-1-positive BAL lymphocytes was 42% or less. In conclusion, Ia antigen expression by BAL lymphocytes could be useful for monitoring rejection in lung allotransplantation.