Recombinant homologues of human proteins have the potential to induce an immunogenic response when used therapeutically. Each of the three interferon (IFN) beta therapies currently approved for multiple sclerosis can induce the development of neutralising antibodies, the full effects of which on IFN beta therapy remain unclear. To investigate the immunogenicity of the currently licensed formulation of subcutaneous IFN beta-1a, 22 or 44 microg three times weekly (Rebif), two new formulations of IFN beta-1a (Rebif) New Formulation [RNF]1 and RNF2) have been developed. In this study, the immunogenicity of the current formulation was investigated against RNF1, RNF2 and an IFN beta standard using ex vivo T-cells. Dendritic cells, isolated from peripheral blood monocytes donated by 26 healthy volunteers, were matured in vitro and incubated with the test antigens for 6h. Autologous CD4(+) T-cells from the same donors were added and further incubated before cytokine release was assessed by ELISpot assay and proliferation by [(3)H]thymidine pulse. Secretion of T-cell-derived interleukin-2 was 79%, 66% and 105% in incubations with RNF1, RNF2 and the current formulation, respectively (normalised to secretion with the IFN standard; p<0.05, RNF2 versus the current formulation). Secretion of IFN gamma was highly variable between donors, with no significant difference observed between formulations. Normalised values for T-cell proliferation were 56%, 44% and 88% with RNF1, RNF2 and the current formulation, respectively (p<0.05, RNF2 versus the current formulation). The immunogenic potential of RNF2 is significantly lower than that of the current formulation when tested ex vivo. This result is now being confirmed through ongoing clinical trials.