Flow-cytometric fluorescence energy transfer (FCET) measurements between fluorescently labeled cell surface MHC-I and ICAM-1 molecules indicated similar receptor patterns in the plasma membrane of interferon-gamma (INF-gamma)-treated colon carcinoma cells as those observed earlier at the surface of lymphoid cells. INF-gamma activation significantly increased the density of MHC-I and ICAM-1 proteins in the membrane. This increase in receptor density was accompanied by decreased proximity level of the homo-associated MHC-I receptors. Hetero-association of MHC-I and ICAM-1 molecules was increased by INF-gamma treatment. INF-gamma changed neither hetero- nor homo-association of transferrin receptors. By staining the sphingomyelin/cholesterol-enriched lipid microdomains with fluorescently labeled cholera toxin B subunit, we found an increase in the amount of lipid-raft associated G(M1)-gangliosides due to INF-gamma treatment. Confocal microscopic results and FCET measurements show that MHC-I and ICAM-1 are components of G(M1)-ganglioside containing lipid-rafts and also support an increase in the size of these lipid-rafts upon INF-gamma treatment. (C) 2002 Elsevier Science.