This chapter describes the features of an image analyzer combined with an automatic data acquisition system and compares it with other methods on the basis of sensitivity, reproducibility, and technician time. Neutrophil chemotaxis can be evaluated quantitatively in vitro by several methods, including surface counting, leading-front measurements, and collecting 51Cr-labeled cells in a second filter. The first two techniques are often tedious, and observer fatigue and/or bias could easily influence results. The principal advantages of the image analyzer system over the visual measurement of leading fronts are improved objectivity and reduced operator fatigue. A further advantage is that three indices may be obtained simultaneously from the same filter. The image analyzer system shares the advantages of objectivity with the 51Cr assay but has superior sensitivity and avoids the need for using a radioisotope. The most promising application of the image analyzer chemotaxis system is likely to be in investigations of the low concentrations of chemotactic factors, where increased sensitivity is required.