Young pigs were immunized with the lung-pathogenic bacterium Actinobacillus (Haemophilus) pleuropneumoniae by aerosol or orally using viable and inactivated bacteria. The cellular changes in the bronchoalveolar lavage (BAL) were studied in repeated lavages after the pigs were infected with live bacteria. The nucleated cells in the BAL were differentiated and lymphocyte subsets determined. There were no major differences between the two routes of immunization or between viable and inactivated bacteria. The immunization induced an increase in all lymphocyte subsets studied and in the appearance of plasma cells and lymphoid blasts. The infection did not cause a further increase except in granulocytes. The lack of a booster-type increase in lymphocytes in the BAL might indicate a different immunologic reaction of the lung or that lymphocytes of the BAL do not represent lung lymphocytes in general. The protective effect of the immunization might be deduced from the increase in lymphocytes after immunization but not from the reaction pattern after infection.