We have investigated the signalling properties of the chemokine receptor, CCR5, using several assays for agonism: stimulation of changes in intracellular Ca²⁺ or CCR5 internalisation in CHO cells expressing CCR5 or stimulation of [³⁵S]GTPγS binding in membranes of CHO cells expressing CCR5. Four isoforms of the chemokine CCL3 with different amino termini (CCL3, CCL3(2–70), CCL3(5–70), CCL3L1) were tested in these assays in order to probe structure/activity relationships. Each isoform exhibited agonism. The pattern of agonism (potency, maximal effect) was different in the three assays, although the rank order was the same with CCL3L1 being the most potent and efficacious. The data show that the amino terminus of the chemokine is important for signalling. A proline at position 2 (CCL3L1) provides for high potency and efficacy but the isoform with a serine at position 2 (CCL3(2–70)) is as efficacious in some assays showing that the proline is not the only determinant of high efficacy. We also increased the sensitivity of CCR5 signalling by treating cells with sodium butyrate, thus increasing the receptor/G protein ratio. This allowed the detection of a change in intracellular Ca²⁺ after treatment with CCL7 and Met-RANTES showing that these ligands possess measurable but low efficacy. This study therefore shows that sodium butyrate treatment increases the sensitivity of signalling assays and enables the detection of efficacy in ligands previously considered as antagonists. The use of different assay systems, therefore, provides different estimates of efficacy for some ligands at this receptor.