Although in vitro assays have been widely used to study leukocyte chemotactic migration, finding the best way to quantitate these assays has proven to be an elusive goal. Investigators have usually resorted to reporting quantities such as the leading front distance, total migrating cells or number of cells past a given distance from their starting point. While these measures may often provide a valid comparison of cell migration under specific assay conditions, they also reflect physical characteristics of the assay that are irrelevant to the basic phenomenon of interest; thus, typical quantities measured in the assay are not useful for comparison between different systems or for correlation with in vivo performance. Recently, however, Tranquillo et al. (1988) demonstrated the utility of an analysis of the under-agarose assay in which the density profile of migrating cells is characterized in terms of two parameters: the random motility coefficient, mu, and the chemotaxis coefficient, chi. These parameters do reflect intrinsic cell movement independently of extraneous physical conditions. The analysis relies primarily on matching theoretical cell density profiles, calculated from a mathematical model in which mu and chi appear, to cell density profiles measured experimentally in the assay. In this paper, we extend the work of Tranquillo et al. to show that the same model can be applied successfully to the Millipore filter assay. In addition, we present measured values of mu and chi for rabbit polymorphonuclear leukocytes (PMNs) in response to, and as a function of the concentration of, the peptide attractant formyl-norleucyl-leucyl-phenylalanine (FNLLP). We also examine the relationship between results obtained for the filter assay and the under-agarose assay and consider the mechanistic basis of the parameters mu and chi.