A neutrophil chemotaxis assay was developd which permits rapid, quantitative assessment of migration across a membrane filter. The critical factor in the assay was the use of a 10 microns thick polycarbonate membrane without the usual polyvinylpyrrolidone coating. Migrated neutrophils remain adherent to the uncoated membrane, whereas 20-50% fall off polyvinylpyrrolidone-coated membranes. A major advantage of the method is that neutrophil chemotaxis can be readily quantified, since the migrated cells adhere to the membrane surface and are in one optical plane for counting. A 25 mm x 80 mm membrane sheet was used in a 48-well micro chemotaxis assembly, which requires only 20,000 neutrophils and 25 microliters of attractant per assay well. Neutrophil chemotaxis was complete within 10-20 min at 37 degrees C, with 20-30% of the cells migrating to N-formyl-methionyl-leucyl-phenylalanine and 40-50% migrating to complement derived C5a.