A flow cytometric procedure was used to analyse the cell surface interleukin-6 receptor (IL-6R) based on the principle of detecting the binding of IL-6 to IL-6R. The bound IL-6 was visualized by reacting with anti-IL-6 antibody, a second biotinylated antibody to immunoglobulins, fluorescein-conjugated avidin and biotinylated, fluorescein-conjugated bovine serum albumin. Studies with a number of human myeloma cell lines showed that the fluorescence signal from cellular binding of IL-6, and thus IL-6R, could be specifically discerned from the background. The specific IL-6R signal could be semi-quantitatively expressed as the mean channel number of the fluorescence histogram or as relative IL-6R density in relation to a reference cell line, such as U266-BL cells. The relative IL-6R density of various myeloma cell lines thus determined was found to correlate with their sensitivity to the growth inhibitory effect of glucocorticoids in vitro. Quantitatively, calibration of the staining procedure with microbeads that have a defined binding capacity for anti-IL-6 antibody allowed calculation of cellular IL-6R density that yielded results close to that reported with conventional radio-ligand binding assay. Similarly, the cytometric method was applied to the studies of kinetics of IL-6/IL-6R interaction and the cellular IL-6R changes after ligand-binding to provide estimates of IL-6R binding affinity and IL-6R turnover rate. It is suggested that flow cytometric measurement of cellular IL-6R is useful in providing biologically relevant information on myeloma cells.