The immunoperoxidase slide assay (ISA) is described as a new method for the demonstration of surface markers on bronchoalveolar lavage (BAL) cells. This technique makes the unlabelled antibody enzyme method using the peroxidase-antiperoxidase (PAP) immune complex for labelling suitable for the evaluation of single cells. By this method, viable cells are firmly attached to poly-L-lysine coated reaction areas of siliconized glass slides. The ISA technique was found to be very useful for testing a battery of monoclonal antibodies against surface antigens on BAL cells from one specimen in parallel. This is due to the low amount of cells needed for this method. Other advantages compared to immunofluorescence techniques are the reduced costs, the possibility of a permanent documentation of the staining on the glass slide preparations, the higher sensitivity and the well preserved morphology of cells. Normal values for lymphocyte markers (OKT3/Leu-1, OKT4/Leu-3a, OKT8/Leu-2a, Leu-7, B1) and monocyte/macrophage markers (OKIa, anti-Mono, anti-IgG) are reported.