Immunocytochemistry using monoclonal antibodies to beta/A4 protein was applied to study the macrophages involved in the removal of amyloid deposits. The material examined included necrotic brain tissue areas with abundant amyloid deposits collected from 32 autopsy cases. The beta/A4-immunoreactive products were found in numerous macrophages, appearing as early as 24 h after the onset of stroke. Immunogold electron microscope studies allowed us to localize the reaction product to the secondary lysosomes. Our study clearly demonstrates the differences between macrophages engaged in amyloid removal and microglial cells associated with amyloid deposits, which according to previous observations contain beta/A4 material within endoplasmic reticular channels.