Chemokines are proinflammatory cytokines that play a role in leukocyte migration and activation. Recent reports showed that RANTES (regulated on activation normal T-cell expressed and secreted chemokine), eotaxin, macrophage-derived chemokine (MDC), and stromal cell–derived factor-1 (SDF-1) are NH₂-terminally truncated by the lymphocyte surface glycoprotein and protease CD26/dipeptidyl peptidase IV (CD26/DPP IV). Removal of the NH₂-terminal dipeptide resulted in impaired inflammatory properties of RANTES, eotaxin, MDC, and SDF-1. The potential CD26/DPP IV substrate macrophage inflammatory protein–1β (MIP-1β) and the related chemokine, LD78α (ie, one of the MIP-1α isoforms), were not affected by this protease. However, CD26/DPP IV cleaved LD78β, a most potent CCR5 binding chemokine and inhibitor of macrophage tropic human immunodeficiency virus–1 (HIV-1) infection, into LD78β(3-70). Naturally truncated LD78β(3-70), but not truncated MIP-1β, was recovered as an abundant chemokine form from peripheral blood mononuclear cells. In contrast to all other chemokines processed by CD26/DPP IV, LD78β(3-70) had increased chemotactic activity in comparison to intact LD78β. With a minimal effective concentration of 30 pmol/L, LD78β(3-70) became the most efficient monocyte chemoattractant. LD78β(3-70) retained its high capacity to induce an intracellular calcium increase in CCR5-transfected cells. Moreover, on CCR1 transfectants, truncated LD78β(3-70) was 30-fold more potent than intact LD78β. Thus, CD26/DPP IV can exert not only a negative but also a positive feedback during inflammation by increasing the specific activity of LD78β. CD26/DPP IV–cleaved LD78β(3-70) is the most potent CCR1 and CCR5 agonist that retains strong anti–HIV-1 activity, indicating the importance of the chemokine-protease interaction in normal and pathologic conditions.