Abstract: A method for the non-radioactive determination of circulating red cell volume, total blood volume and red cell survival. Red blood cells are biotinylated and injected into the subject for dilution in the subjects total blood volume. A diluted sample is extracted and incubated with a label, either a radionuclide or a fluorescent moiety complexed with avidin or streptavidin. Detection of the label, as for example by gamma counting in the case of a radionuclide or fluorescence activated cell sorting in the case of a fluorescent moiety, allows calculation of the red cell volume and therefrom, total blood volume. Sequential measurements are possible with this method. Aspects of the method include enhanced biotin labeling and separation of cells by density gradient means.