The neutrophil shape change response to a chemotactic formylpeptide was assessed. Neutrophil bipolar shape formation (BSF) was also simultaneously assessed with a Boyden chamber-based neutrophil migration assay. Both assays were precise and relatively reproducible; the average coefficient of variation for the BSF assay was 9.6% and 9.2% for the migration assay. In a blind study the BSF assay showed 100% sensitivity at detecting subjects with known abnormal neutrophil migration. Unlike the migration assay, the BSF assay does not require isolated neutrophils, reducing possible cell activation and monitoring the cell response under more physiological conditions. Small blood samples--1 ml or less compared with 20-40 ml for the migration assay--are used, and the method is technically simple. Results are available within 40 minutes, and routine (EDTA) blood samples are used. It is concluded that the BSF assay is a suitable motility screening test for both the clinical and pharmacological examination of the movement of polymorphonuclear leucocytes.