Immunophenotyping of lymphoid tissues, particularly spleen and thymus, has been used in the field of immunotoxicology for many years as part of a tiered screening approach for detecting immunotoxicity following chemical exposure. This unit describes basic procedures for direct and indirect immunofluorescent antibody staining of surface proteins on lymphoid cells from tissues or blood. Protocols for resolution of dead cells and for fixation of cells are also included. In addition, information on antibody selection and titration, fluorochrome choices, isotype controls, compensation, standardization of flow cytometric analysis, data analysis, and statistical issues is presented.