The incidence, distribution and chemical composition of amyloid-beta (A beta) peptide-positive deposits were investigated in the lower primate species common marmoset (Callithrix jacchus). No A beta deposits were observed in the brains of 7 marmosets below 7 years of age. In 15 marmosets above 7 years, 60% displayed cortical A beta-immunoreactive plaques, 80% had A beta deposited in intracortical vessels and 87% displayed A beta deposits in meningeal vessels. The cerebral cortex of the oldest animal (15 years) contained a substantial density of deposits. A beta-immunoreactive plaques were found predominantly in association cortical zones followed by a lower density in paralimbic cortical areas. Deposits within vessels were most frequent in occipital cortex. A beta40 was found primarily in vascular deposits, while A beta42 was present in plaques. Approximately 20% of plaques and most vascular deposits displayed thioflavin S staining, indicative of the presence of fibrillar A beta. Varying proportions of A beta deposits contained acetylcholinesterase or butyrylcholinesterase activities and apolipoprotein E and alpha1-antichymotrypsin immunoreactivity. A few plaques contained immunoreactivity for amyloid precursor protein in swollen neurites. However, no abnormally phosphorylated tau immunoreactivity was present in these neurites. Survival analysis in a colony of marmosets indicated that only 6% of animals can be expected to survive beyond 7 years of age. These results indicate that the aged marmoset brain displays A beta deposits with a distribution and chemical composition similar to those found in the human. These similarities suggest that the aged marmoset may be a useful lower primate model for the study of the pathological effects of A beta. However, the relatively small number of animals which can be expected to reach old age severely limits the utility of this species as a model of A beta deposition.