Calcium fluxes of human neutrophils measured in the presence of chemotactically active serum showed a marked stimulation of calcium-45 uptake from the media. Chemotactically inactive serum did not cause an influx of calcium. The magnitude of the calcium influx due to activated serum is sufficient to trigger contractile systems previously described in muscle cells. Lanthanum chloride inhibited the chemotactic response of human neutrophils to activated serum. Lanthanum in concentrations that suppressed chemotaxis also inhibited the calcium influx caused by activated serum. Lanthanum in concentrations that suppressed chemotaxis also inhibited the calcium influx caused by activated serum. The data support a direct role of calcium influx in chemotaxis of neutrophils.