Occurrence of the classical pathway complement proteins C1q, C1r, C1s, C2, C3, C4, C5, C6, C7, C8 and C9 was studied in human hippocampus and temporal cortex by immunohistochemistry and Western blotting. In Alzheimer disease (AD) cases, positive staining for all of these proteins was observed in pyramidal neurons and senile plaques. In control cases, weaker pyramidal neuron staining was observed except for C1q and C1s which were not detected. On Western blots of AD hippocampal extracts, bands corresponding to those detected in normal serum were found for each of the complement proteins. Comparable bands were also detected in normal hippocampal extracts with the exception of C1s which was not observed. The intensity of the bands was generally stronger in AD than in normal extracts, but, in the latter, there was considerable variability between cases and between bands in a single case. These data suggest that pyramidal neurons may be a source of the complement components known to be associated with Alzheimer lesions.