CC-chemokine receptor 3 (CCR3)-stimulating chemokines are likely to have important in vivo roles in the regulation of eosinophil, basophil, and potentially helper T cell type 2 and mast cell recruitment. We have developed techniques to investigate the actions of eotaxin and other chemokines on multiple leukocyte populations in whole blood, without cell purification steps that might alter leukocyte responsiveness. We have shown that the potency of eotaxin in whole blood is limited by Duffy antigen binding, which may modulate the actions of this chemokine in vivo. We have also investigated the efficacy and potency of a new panel of small molecule antagonists of CCR3 on responses of eosinophils, neutrophils, basophils, and monocytes to chemokines, using whole blood assays of shape change, chemokine receptor internalization, and CD11b upregulation. These small molecule antagonists cause selective and potent inhibition of CCR3 on eosinophils and basophils, are bioavailable in blood, and are prototypic antagonists potentially of benefit in the treatment of human allergic disease. Such whole blood methods may also be employed in the investigation of other small molecule chemokine receptor antagonists.