This assay is designed to enumerate percentage and absolute cell counts of lymphocyte subsets in whole blood sample. Whole blood is added with fluorochrome-labeled antibodies that bind specifically to cell surface antigens on lymphocytes. After incubation, lysing and fixation, percent and absolute lymphocytes counts are enumerated utilizing an internal quantitation standard. Antiobodies used include CD3, CD4 and CD8 (for T-cells), CD56 (for NK cells), and CD19 (for B-cells).