The CAT1 receptor mediates ecotropic gammaretrovirus entry and the cytopathic response to virus infection. Use of virus env variants, receptor mutations, and inhibitors of glycosylation demonstrate that both of these virus-receptor interactions are modulated by a small number of critical amino acid residues in virus and receptor, and that Nlinked glycans can modulate entry for specific virus-cell combinations. Virus stocks were made by collecting culture fluids from infected or transfected cells. These stocks were titered by the XC overlay test [9] following infection of NIH 3T3, SC-1 [24] , or M. dunni [5] and cells transfected with CAT-1 receptor. Cells were plated at 1-2 × 10 5 cells/60 mm dish and infected with 0.2 ml of appropriate dilutions of virus stocks in the presence of polybrene (4 ug/ml; Aldrich, Milwaukee, WI). Cells were irradiated 4 days after virus infection with ultraviolet light from germicidal bulbs (30 sec at 60 ergs/mm 2 ) to kill the cells but not the virus, and were then overlaid with 10 6 XC cells/plate. XC cells produce plaques containing syncytia in response to focal areas of virus infected cells. Plates were fixed and stained 3 days later and examined for plaques of syncytia.