We have examined the interaction of fluorescein isothiocyanate (FITC) labeled E. coli lipopolysaccharide (LPS) with human T lymphocytes by means of cell flow cytometry, in an effort to establish the type and the conditions of this interaction. Our results indicate that the interaction of LPS with human T lymphocytes is optimal at 37 degrees C. This interaction is rapid and reaches the maximum after 5 minutes and is followed by endocytosis. Our results show that sodium azide and trypsin treatment does not affect LPS- human T cell interaction, suggesting a nonspecific lipid-lipid interaction.