The present article reviews findings regarding the age-related occurrence of clusters of unusual granules in the brains of C57BL/6 (B6) mice and discusses the potential relevance of this phenomenon as a model of specific aspects of brain aging in humans. The granules occur predominantly in the hippocampus of B6 mice and represent aggregations of fibrillar material that are mostly associated with astrocytes. The deposits become evident at about 4 to 6 mo of age, and increase markedly in both number and size thereafter. Similar structures have been observed in adult senescence accelerated mice (SAM) and have been noted, although very rarely, in older mice from other strains. The deposits appear to manifest dominant genetic heritability. Heparan sulfate proteoglycan and laminin or related molecules have been identified as components of the granular material. Although the deposits do not represent senile plaques with beta-amyloid deposition, they might mimic the deposition of extracellular matrix molecules that is thought to be an early event in amyloidogenesis in the aged brain and in Alzheimer's disease.