The absolute number of membrane-expressed CD14, the most important endotoxin receptor, on human monocytes and neutrophils shows remarkable variation in the literature. To quantify these numbers two fluorescence methods using fluorescein isothiocyanate (FITC)-labeled monoclonal antibodies (mAb) were applied. A commercially available set of standard beads was used in flow cytometry to quantitate CD14 with eight different mAbs. Independent from their isotype the various mAbs showed minor differences and indicated that peripheral blood monocytes expressed 99,500-134,600 (115,400 +/- 10,600) and neutrophils 1,900-4,400 (3,300 +/- 800) CD14 receptors. There was no significant difference in CD14 expression on leukocytes in unprocessed freshly obtained whole blood and after a Ficoll isolation procedure. However, a short temperature shift resulted in a 1.3- to 1.6-fold up-regulation of CD14. The results obtained with the reference beads were verified with fluorescence Scatchard analysis and spectrofluorometry using mAb 26ic-FITC and showed 109,500 CD14 per monocyte and 6,700 CD14 per neutrophil. For comparison the number of CD14 on the monocytic THP-1 cells and Fc gamma-receptors on human leukocytes were determined using the reference beads and flow cytometry and gave results comparable to published data. Our data indicate that resting human monocytes express roughly 110,000 CD14 molecules on their surface using a simple fluorometric assay. Correct determination of the number of CD14 and other cell surface receptors is of importance in the monitoring of septic patients.