FCM has become an invaluable technique in a variety of clinical and research applications. Then, FCM is used for quantitative analysis of human erythrocyte antigens using Spectrum III. Antibody sensitizations give rise to erythrocyte agglutination making the FCM analysis of a single cell impossible. However, such agglutination could be prevented by some of erythrocyte fixations with PFA and DMS. To make examinations in an optimal concentration of primary antibodies was important for FCM assay. FCM was used to detect small percentage of D antigen positive cell contaminated in negative samples, subsequently positive cells were finally detected in 0.2%. This assay was utilized on a recipient of M/N mismatched allogeneic BMT, and an accurate proof of engraftment was obtained. Relationship of the numbers of antigen determinants and immunofluorescence intensities might be in linear correlation by log-log transformation. The fluorescence intensities of D antigen were analyzed for different rhesus phenotypes and variants such as Du and -D- cells. Lower fluorescence intensities on Du and higher intensities on -D- cell could be seen comparing with average value of D positive cells. However, no relationship between rhesus phenotypes and anti-D immunofluorescence intensities was observed. These findings suggest that FCM is useful and accurate technique for measuring relative antigen determinants of erythrocyte.