To evaluate the value of a bronchoalveolar cell profile in the early detection of canine lung allograft rejection, bronchoalveolar lavages were done serially in mongrel dogs before and after single lung transplantation. The dogs were divided into 3 groups. In group 1, neither donor nor recipient dogs were treated with cyclosporine. In group 2, only donors were treated with cyclosporine, orally at a single dose of 20 mg/kg/day for 3 days prior to single lung transplantation. In group 3, only recipients were treated with cyclosporine (20 mg/kg/day) for 9 days after single lung transplantation. A marked increase in the number of bronchoalveolar cells and their cell differentials, and of major histocompatibility complex (MHC) class II-positive cells obtained from the grafted lungs after lung transplantation, was seen in groups 1 and 2. The changes in bronchoalveolar cell profile obtained from the rejecting grafted lungs were significantly different from those obtained from the normal and native lungs (P < 0.05). In group 3, the bronchoalveolar cell profile obtained from the grafted lungs did not significantly differ from those present in normal and native lungs during the period of cyclosporine treatment after lung transplantation. On various days after withdrawal of cyclosporine treatment, bronchoalveolar cell profiles obtained from the grafted lungs showed similar changes to those observed in groups 1 and 2. Abnormal changes in bronchoalveolar cell profiles obtained from the grafted lungs heralded the appearance of abnormalities detected by chest X-ray films. Our results indicate that serial measurements of bronchoalveolar cell profile may serve as a useful means for early detection of canine lung allograft rejection.