The Genetic Systems Technique (GS: direct immunofluorescence microscopy with ethidium bromide counterstaining of nuclei) was tested for quantitative analysis of T-lymphocyte subsets in human peripheral blood. The monoclonal antibodies anti-CD4 and anti-CD8 were used for detection of T-helper and T-suppressor cells respectively and the results compared to those obtained by conventional indirect immunofluorescence microscopy and the Technicon Enzyme Immunoassay (EIA) with automated reading. The GS technique provided results correlating well with both indirect immunofluorescence and EIA techniques. Moreover, this method has two advantages: it is less time-consuming than the indirect immunofluorescence microscopy and necessitates less expensive and more commonly available equipment than the automated EIA technique.