Neuronal loss in Creutzfeldt-Jakob disease (CJD) is due to apoptosis and autophagy, however the apoptotic cell death may be related to the influence of cytokines. The role of complement system in induction of this process was recently postulated. In our study we examined the immunoreactivity for "early activation" complement factors C1q and C3b in the brains of the patients with CJD. We showed the positive immunoreactivity for C1q and C3b in neurons mainly in areas of spongiform change, in Purkinje cells, molecular layer and granular layer of cerebellum, and in prion protein positive plaques. The evident positive reactivity of two complement factors may suggest the role of complement system in neurodegeneration in CJD.