The epidermal growth factor receptor (EGFR) is considered a tumor-related marker with potential diagnostic and prognostic value. In order to assess the sensitivity of flow cytometry to detect EGFR and to quantify receptors objectively, two human bladder carcinoma cell lines with different urothelial differentiation, RT4 and J82, were grown in vitro, and their membrane EGFR content was measured by flow cytometry. Exponential monolayers showed decrease of EGFR content after 20 min pulses with 10 ng/ml EGF in medium, as detected with the antibody EGFR1 in a double staining technique with propidium iodide for DNA evaluation. Further decrease of green fluorescence intensity was seen in cells constantly exposed to EGF. Absolute receptor numbers were determined by Scatchard analysis with radioactive EGF and resulted in relatively low receptor numbers for both cell lines (approximately 3-4 x 10(4) EGFR/cell), as well as one affinity class. These findings could be matched by absolute receptor quantification by flow cytometry, adding beads with defined antigenic sites (Quantum Simply Cellular, Microbead Corporation) to the cell suspension for staining. Our data suggest that flow cytometric EGFR detection and quantitation may be supplied to in vivo tumor samples and that measurements by multiparameter analysis may define subpopulations valuable for tumor diagnosis and judgment on tumor progression.