Bronchoalveolar cells from 13 lavages done in six heart-lung transplant recipients were tested for spontaneous proliferation, Interleukin-2 response and secondary allogeneic proliferation measured by primed lymphocyte testing. In most instances the bronchoalveolar cells responded to Interleukin-2 suggesting the presence of activated T-cells in the lavage. The lavage cells from several patients exhibited primed lymphocyte testing responses suggesting the presence of alloreactive T-cells. A high degree of spontaneous cellular proliferation was observed in several lavages, which appeared to be due to a macrophage-lymphocyte reaction. Two mechanisms may be responsible for this reactivity; a macrophage-lymphocyte reaction as a result of recipient lymphocyte responses to donor macrophages in the lavage during the early post-transplant period, or a macrophage-lymphocyte reaction related to an ongoing infection. Thus, functional assays on bronchoalveolar lavage cells may have diagnostic value in monitoring heart-lung recipients for lung rejection and infection.