We have studied the impact of short-term treatment with interferon (IFN)-beta1b of relapsing remitting (RR) multiple sclerosis (MS) patients' blood levels of type 1 and type 2 cytokines such as IFN-gamma, interleukin (IL)-1beta, IL-2, IL-4, IL-6, IL-10 and tumour necrosis factor (TNF)-alpha. These cytokines were measured by solid-phase ELISA. Serum samples were obtained prior to, and 2 and 12 hours after beginning of the treatment and 48 h after the last of 5 s.c. injections with 8 million IU IFN-beta1b given on alternate days for 10 days. The treatment was found to increase the circulating levels of IL-2, IL-6, IL-10 and IFN-gamma at some of the time points considered, with the effect acquiring statistical significance for IL-6, IL-10 and IFN-gamma. The blood levels of IL-1beta, IL-4 and TNF-alpha remained below the limit of sensitivity of the assays at any of the time points considered. If this in vivo study mirrors the impact of IFN-beta1b on MS patients' immune cells, these data demonstrate an activation of the immune system upon early treatment with the drug that does not lead to either type 1 or type 2 cytokine prevalence.