Objectives: We examined the effect and time of addition of β-chemokines on human immunodeficiency virus type 1 (HIV-1) replication, binding, and uncoating in human macrophages and measured CCR5 receptor expression during virus binding and uncoating. Methods: Macrophages were treated with β- chemokines before infection, at infection, or postinfection, and virus replication was determined by p24 antigen level. Binding and uncoating of 35[S]-methionine-labeled HIV-1 was measured. CCR5 expression was determined by flow cytometry. Results: The β-chemokines potently inhibited virus replication. The strongest inhibition occurred when cultures were pretreated and maintained with β-chemokines. β-Chemokines also caused strong inhibition of viral uncoating and a considerable decrease in CCR5 expression during uncoating. Conclusions: CCR5 receptors appear to be internalized and recycled to the cell surfaces during HIV entry. The down-regulation of CCR5 expression by β-chemokines during virus uncoating probably accounts for the reduction in virus uncoating (entry) and hence in virus replication. (C) Lippincott Williams and Wilkins, Inc.