The immunoalkaline phosphatase procedure is described as a method for labelling bronchoalveolar lavage cellular specimens with monoclonal antibodies. This method has several advantages over conventional immunofluroescent techniques: it can be performed on cytocentrifuge preparations stored for long periods before staining; cell morphology can be observed in detail in positive and negative cells; the staining is permanent and stable, and, the reaction can be evaluated with a light microscope. Normal values for lymphocyte subpopulations in smokers and nonsmokers are also reported.