Ultrastructural, three-dimensional reconstruction and morphometric studies of classical plaques from the cortex of a patient with Alzheimer's disease showed five or six microglial cells, which form, together with the amyloid star, the central complex of the classical plaque. Microglial cells associated with the amyloid star show marked polymorphism, but all forms possess an amyloid making pole. The surface of the cell membrane at this pole is extended by apparent connection with membranes of cytoplasmic channels filled with amyloid fibers. The amyloid pole also shows other features of local activation with nuclei translocation, expansion of Golgi apparatus and endoplasmic reticulum, and multiplication of vacuoles and coated vesicles that are in close proximity to channels filled with new polymerized amyloid fibers. On the basis of ultrastructural studies, three forms of microglial cells can be distinguished: macrophage-like, cap-like, and octopus-like cells. The most effective in production of amyloid fibers seem to be cap-like microglial cells, which have the greatest interface with the amyloid star. Octopus-like cells have the least contact with the amyloid star. The size of the surface of the interface with the amyloid star appears to be an indicator of the extent of cell engagement in amyloid fiber formation.