Abstract: A flow cytometry method for reproducibly detecting and counting a lymphocyte population of interest in a leukocyte suspension or whole blood sample in which the red cells are subsequently lysed. The suspension (or sample) is combined with a reagent comprising a primary antibody, either native, carrying an attached enzyme or biotin or other label, and a fixative reagent, in either order. Where the enzyme is not attached, an enzyme is coupled specifically to the primary antibody. The fixed suspension is reacted with a color-producing enzyme-cytochemical reagent. The suspension, now including stained and unstained fixed cells, is passed through a flow cytometer and the cells are characterized and counted on the basis of their light-scattering and light-absorbing properties.