Polyethylene glycol (PEG) induces rapid fusion of LM cells. Membrane fusion, as detected by formation of pentalaminar membrane arrays, occurs as early as 1 min after PEG treatment. The entire cell surface arrears to be capable of fusion since fusion occurs in regions where pseudopodia make contact with each other or with a neighbouring cell body and also in areas where cells are in contact along their entire periphery. Cytoskeletal components showed no apparent deleterious effect from PEG treatment or subsequent cell fusion as determined by thin-section EM. Freeze-fracture of monolayer cultures reveals a thermotropic rearrangement of intramembranous particles following PEG treatment.