Frozen stored human mononuclear cells were compared with non-frozen cells as responder cells in the monocyte chemotaxis assay. Frozen stored cells were found to inhibit lower background migration, resulting in greater experimental-to-control ratios. The use of frozen cells also greatly improved the speed, standardization of conditions and day-to-day reproducibility of the assay procedure. Frozen cells were at least as sensitive as fresh cells for detecting low chemotactic factor concentrations and were superior for detecting differences in concentration.