Both amyloid beta protein (A beta) and aluminum (A1) have been implicated in Alzheimer's disease. Recently, A beta has been found to be produced by peripheral tissues as well as by the CNS and to cross and accumulate in the vascular bed of the brain, which comprises the blood-brain barrier (BBB). This raises the possibility that blood-borne A beta may be a source of A beta within the CNS. Al has been shown to alter the structure and function of A beta, to inhibit the class of enzymes (metalloproteases) associated with the processing and degradation of A beta, and to alter the permeability of the BBB to peptides of similar size to A beta. Therefore, Al could alter the access of blood-borne A beta to the CNS either by changing the permeability of the BBB or by affecting enzymatic degradation. We examined the effect of Al on both of these parameters and found that Al did not alter the permeability of the BBB to A beta radioactively labeled with 125I (I-A beta) ever after correction for in vivo degradation. However, Al did enhance clearance and degradation of I-A beta in the circulation but not in the brain. Alterations in clearance can indirectly affect the CNS accumulation of circulating substances by modifying their presentation to the brain. In vitro studies of intracellular enzymatic activity of lysates of mouse and human erythrocytes (RBC) showed that Al could inhibit degradation of I-A beta through a mechanism antagonized by calcium and dependent on the concentrations of RBC lysate and Al. Analysis by high performance liquid chromatography showed that Al acted primarily by inhibiting the initial degradation of I-A beta to a peptide intermediate without inducing the aggregation of I-A beta under the conditions of these studies. No difference was found in sensitivity to Al between RBCs from patients with Alzheimer's disease and age- and sex-matched controls. The ability of Al to alter the degradation of A beta suggests a way in which these two potentially neurotoxic substances might interact in conditions such as Alzheimer's disease.