The influence of interferon (IFN)-β on cytokine release by immune cells remains controversial. This study compared IFN-β1b effects on mononuclear cells, CD4+ and CD8+ T cells derived from healthy controls and relapsing–remitting multiple sclerosis (RRMS) and primary progressive multiple sclerosis (PPMS) patients. Effects of IFN-β1b (0-10,000 U/ml) on cytokine release were determined in cell culture. IFN-β1b inhibited IFN-γ and induced interleukin (IL)-4 selectively in RRMS-derived CD4+ T cells. IL-10 was significantly induced in all cell populations from RRMS but only marginally in PPMS. IL-5 was always inhibited; IL-17A remained unaltered. These in vitro data parallel clinical observations that IFN-β is most effective in RRMS.