Subsegmental bronchoalveolar lavages were performed in 18 healthy beagles. The average yield per lavage was 45 X 10(6) cells consisting on the average of 24% lymphocytes, 71% macrophages, and 4% granulocytes. Cells were further examined in cytofluorometric studies using monoclonal (anti-Ia, antilymphocyte, anti-T) and polyspecific (anti-Ig) antibodies. Sixty to 90% of lymphocytes were T cells as determined by the T cell antibody DT-2. No surface immunoglobulin-positive cells (B cells) could be detected. All macrophages expressed Ia antigens (p 29/34) whereas lymphocytes did not. In assays of concanavalin A-induced blastogenesis of thymocytes, alveolar macrophages functioned as accessory cells. The anti-Ia antibody 7.2 interfered with this function, indicating that Ia antigens on canine alveolar macrophages are involved in interaction with T cells resulting in T cell proliferation.