Focal injection of lysolecithin into the thoracic spinal cord was used to induce experimental demyelination in rats. The macrophages which invaded the demyelinating area to phagocytose the myelin breakdown products were examined by electron microscopy using conventional thin-section and freeze-fracture techniques. The cells had the typical foamy cytoplasm characteristic of lipid macrophages. In freeze-fracture preparations, the P-face of the plasmalemma had scattered 10-18 nm round to oval or elongate particles. The cytoplasm was filled with phagocytic vacuoles containing myelin debris. Occasionally, membranous connections could be demonstrated between lysosomes and phagocytic vacuoles. The debris in the vacuoles varied in structure from material with a normal myelin periodicity to partially digested lamellated structures and pseudocrystalline structures to homogeneous-like, lipid-filled vacuoles. These patterns appeared to represent successive stages in the digestion of the ingested myelin.