Analysis of ligand-receptor interactions with the fluorescence activated cell sorter

The binding of a fluorescent peptide to human neutrophils is analyzed with a fluorescence activated cell sorter. We examine steady-state and kinetic features of the ligand-receptor interaction (in the presence of unbound ligand) and we show tht the number of receptors may be estimated without resorting to any external references for calibration. These methods are applicable to other fluorescent ligands with affinities greater than 10(8) M-1.