In cases of retinal light damage, glaucoma, or senile macula degeneration, the loss of retinal neurons is thought to cause alterations of glial cells. We performed immunocytochemical studies on retinae of (i) healthy rats and human donors, (ii) rats exposed to enhanced illumination for 24 months, a procedure which leads to complete loss of photoreceptor cells, (iii) a human donor who had suffered from senile macula (photoreceptor cell) degeneration, and (iv) human donors who had suffered from glaucoma, known to be accompanied by a loss of ganglion cells and other retinal neurons. Furthermore, Müller cells were enzymatically isolated from human glaucomatous retinae. All preparations were subjected to immunocytochemistry for CD44 antigen and Apolipoprotein E (ApoE). In normal rat and human retinae, CD44 immunoreactivity was observed in the microvillous sclerad processes of Müller cells: in human retinae, perivascular (astro-)glial cell processes were also CD44 immunopositive. ApoE immunoreactivity was only found in some perivascular (astro-)glial cell processes of human retinae. Both rat and human Müller cells respond to photoreceptor cell damage by increased, and ectopic, expression of the CD44 antigen. Increased ApoE immunoreactivity was found in Müller cells from degenerative human retinae, but rarely in light-damaged rat retinae. It is concluded that degeneration-related reorganization involves enhanced expression of the glial cell adhesion molecule CD44 as well as elevated activity of the glial lipid transport molecule ApoE.