Treatment with interferon (IFN)-beta reduces clinical disease activity in multiple sclerosis (MS). Using flow cytometry, an enzyme-linked immunosorbent assay and a real-time polymerase chain reaction, we studied in vivo IFN-beta-induced effects on CD4(+) T-lymphocyte chemokine receptor expression as these influence central nervous system (CNS) transmigration and inflammation. At 'steady state' (>/=1 day after the most recent IFN-beta injection), IFN-beta treatment increased CD4(+) T-cell surface expression of CC chemokine receptor (CCR)4, CCR5 and CCR7 after 3 months of treatment, whereas that of CXC chemokine receptor (CXCR)3 was unaltered. Conversely, at 9-12 h after the most recent IFN-beta injection, CCR4, CCR5 and CCR7 expressions were unaltered, while CXCR3 expression was reduced. CD4(+) T-cell surface expression of CCR4 was significantly lower in untreated MS patients compared with healthy volunteers. Of the plasma chemokines, only CXCL10 was increased by IFN-beta treatment; CCL3, CCL4, CCL5 and CXCL9 were unaltered. CCR5 mRNA expression in blood mononuclear cells correlated with the expression of T-helper type 1 (Th1)-associated genes whereas CCR4 and CCR7 mRNA expression correlated with Th2 and immunoregulatory genes. In conclusion, IFN-beta treatment caused 'steady-state' increases of several chemokine receptors relevant for CD4(+) T-lymphocyte trafficking and function, possibly facilitating lymphocyte migration into the CNS. An important therapeutic effect of IFN-beta treatment may be the normalization of a decreased Th2-related CD4(+) T-cell CCR4 expression in MS patients. Surface chemokine receptor expression and CXCL10 varied according to the timing of blood sampling in relation to the most recent IFN-beta injection. Thus, it is imperative to distinguish acute effects of IFN-beta from steady-state effects.