This study examined Wallerian degeneration (WD) of rat optic nerve (ON) with 2 goals: to determine which cell types are involved in myelin degradation and clearance and to evaluate the extent to which Ia antigen, a product of the immune response genes, is expressed. We examined immunostained 1-micron and ultrathin cryosections of rat ON at 1, 8, and 16 weeks after nerve transection. Serial 1-micron cryosections were stained with monoclonal antibodies to rat Ia antigen (Ox6) and with antibodies that identify astrocytes (GFAP) and monocytes/macrophages (ED1). In normal ON, ED1-positive cells were not found. A few ED1-positive monocytes/macrophages were present in the transected ON at one week. Macrophages were prominent throughout the ON at 8 weeks; by 16 weeks their number was decreasing. These cells contained myelin debris in various stages of digestion. GFAP-positive astrocytes did not contain myelin debris in their cytoplasm. At all 3 times a subpopulation of the ED1-positive monocytes/macrophages expressed Ia antigen. Ia antigen was not detected in endothelial cells or GFAP-positive astrocytes. In ultrathin cryosections stained by immunogold procedures, Ia immunoreactivity was found exclusively in cells containing multiple vacuoles and myelin debris and lacking intermediate filaments. The same cell type was labeled by ED1 antibodies. Our results indicate that macrophages remove the vast majority of debris during Wallerian degeneration in the CNS; a proportion of these macrophages concomitantly express Ia antigen. These results suggest that the Ia expression by macrophages observed in other CNS disorders does not necessarily reflect specific local immune events, but in some instances can represent a nonspecific response to CNS damage.