A method for measuring the responses of sets or subsets of lymphocytes to mitogens or antigens in a sample is disclosed comprising incubating a population of cells with a mitogen or antigen, separating the desired subset of cells by means of the interaction of a specific binding reagent that is attached to the solid phase with a cell surface determinant that is present on the cell subset of interest, lysing the separated cells, and measuring an intracellular component that is increased if the cells have responded to the stimulus. The method provides a convenient, simple, and reliable method for measuring immune function in a variety of conditions.