This unit describes procedures for measuring the activation of chemokine receptors by their ligands. Chemokines are chemoattractant proteins and two assays are described for analyzing the chemoattractant properties (chemotaxis) of these proteins. Chemokine receptors are coupled to G proteins, and activation of the receptors results in the mobilization of intracellular Ca²⁺ stores. Two assays for the measurement of this activity are described which employ calcium-sensitive dyes, one for adherent and one for non-adherent cells. In addition, an assay for quantitating chemokine receptor downmodulation and subsequent recycling events is described. The assays are applicable to both purified leukocytes and stably expressed recombinant chemokine receptors.This unit describes procedures for measuring the activation of chemokine receptors by their ligands.