Most methods of measuring neutrophil motility provide information mainly about the performance of a small proportion of the fastest moving cells. Application of a computer-linked image analysis technique, using the “Quantimet,” provides a convenient, automated method of measuring the motility of the whole cell population. This makes it possible to test whether changes in motility represent a homogeneous alteration affecting all cells or a change in the numbers or performance of a subset of cells. In this study the neutrophils from patients with uncomplicated rheumatoid arthritis were found to perform similarly to normals, while cells from patients with Felty's syndrome were markedly slower. This was an overall, homogeneous slowing of the whole cell population, not due to a loss of fast moving cells.