The five rabbit antirhesus monkey erythrocyte specificities of Owen and Anderson1 have been confirmed. An additional specificity, designated "F," has been found in 91.2 per cent of rhesus animals tested. Antiserum reagents with the A and B reactivities have been used in the indirect fluorescent antibody technic to detect and quantitate minor rhesus erythrocyte populations in admixture with a major population. Specific fluorescence was achieved with wet, unfixed preparations and was "all-or-none." A rapid method for the estimation of minor to major cell population ratios well above 1:1000 is presented.