The presence or quantity of a selected subset of cells, which is part of a subpopulation of a mixed cell population, is determined by a method in which a detectable reporter substance is uniformly incorporated into substantially all cells of the subpopulation containing the subset of interest. The subset of interest is then affinity-separated by incubating a test sample of the mixed cell population containing the labeled subpopulation with a specific binding substance which selectively binds to characteristic determinants of the cell subset of interest. Occurrence of the reporter substance in the separated fraction is then detected, and correlated to a predetermined standard to determine the presence or quantity of the subset of interest within the cell population. According to another aspect of the invention a method is provided for quantitating two or more selected subsets of cells within a subpopulation of a mixed cell population. After labeling, the entire subpopulation is affinity-separated from the mixed cell population, and occurrence of the reporter substance in the separated fraction is detected. Next, subsets of interest within the subpopulation are affinity-separated as described above, and the level of detected reporter substance in each subset is compared to the level detected in the entire subpopulation. According to further aspects of the invention test kits are provided for performing the above-described methods.