Cerebral arteries from 65 human subjects were examined by immunofluorescence, using antisera against human fibrinogen and low density lipoprotein (LDL). Deposition of fibrinogen and LDL was most frequent at the bifurcation of the middle cerebral arteries and least in the basilar arteries in all age groups. In general, deposition of LDL was associated with deposition of fibrinogen, and lone deposition of LDL in the absence of fibrinogen was only rarely seen. Fibrinogen was scattered in the intercellular spaces, and located in the inner layer or edges of the thickened intima of the bifurcation with increasing plaque formation. Fibrinogen was observed even in the subendothelial region of the uninvolved intima at the bifurcations. LDL was present in the cytoplasm of the endothelial cells in the earliest stage, and it increased in the extracellular stroma with increase in intimal thickening, corresponding closely to the distribution of perifibous oil-red-O-stained lipids. No LDL was detected in the uninvolved intima. The observations suggest that deposition of fibrinogen in the intima might precede LDL deposition and possibly play a more important role than LDL in the development of atherosclerotic lesions in the cerebral arteries, especially in their early stage. Severe atherosclerosis at the bifurcations may be in part due to increased permeation of these plasma proteins, possibly as a result of hemodynamic stress.