Immunoperoxidase techniques were used to obtain information about the possible presence of serum factors in senile plaques. We found only in plaques consisting of an amyloid core surrounded by a corona of degenerating neurites small amounts of IgG and light chains (kappa and lambda). These immunoglobulins were principally localized in the corona and not in the central amyloid core. Further it was found that all plaques contain the complement factors C1q, C3b, C3c, C3d and C4. Senile plaques lacked C5, C3 pro-activator and properdin. The possible significance of these findings in the genesis of the senile plaques and amyloid formation is discussed.