Abstract: An assay for simultaneously determining the ratio of B cells and T cells to the total cell population and subpopulations thereof present in a lymphocyte population utilizes excess amounts of B cell binding protein and T cell binding protein bound to solid phase particles. By exposing the particles to the lymphocyte population, rosettes are formed which may be visually distinguished and counted under a microscope, yielding the proportions of B cells and T cells, including any subpopulations for which a specific label was provided. Macrophages may be identified by their ability to ingest the particles.