Aliquots of cell suspensions (3–6 ml, depending on the density of the suspension) were placed in Petri dishes (Ø 50 mm). For assaying the alkalinization response, the pH indicator chlorophenol red (10 mM) was added. For assaying the oxidative burst, cells were supplied with the peroxidase substrate 5-aminosalicylic acid (100 mM). Using a narrow tipped pipette most of the medium was removed, leaving a thin 1–2 mm thick layer of (wet) cells in the dish. Test substances in a volume of 1 ml were applied locally onto the lawn of cells, and the Petri dishes were analyzed optically by a flat-bed scanner at intervals.