We have developed a simple method for comparing the relative fluorescence intensity (FI) of flow cytometry histograms. It entails assessment of the FI (equivalent to the fluorescence-activated cell sorter (FACS) channel) of the 50th or 75th percentiles of either positively stained cells or the total cell population. We illustrate the method with dilution curves of 1) monoclonal antibodies against the T4 surface antigen of human peripheral blood lymphocytes and 2) fluorescent low density lipoprotein (LDL) binding to the human peripheral blood lymphocytes LDL receptor. We demonstrate the versatility of the method by characterizing the binding properties of fluorescent LDL to their receptors. Binding was shown to be specific and of high affinity, and to reach a steady state plateau at about 2 hr; the affinity of fluorescent LDL for the receptor was found to be two to three times higher than that of the unlabeled LDL.

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