Lymphocytes can be subdivided into two major groups according to origin and function (1,2). T lymphocytes are thymus dependent and are responsible for cell-mediated immunity while the B lymphocytes represent precursors of immunoglobulin-producing plasma cells and give rise to humoral immunity. Considerable knowledge concerning the biological properties of T and B lymphocytes and their mutual interaction has been obtained in the last few years. Recent data also demonstrate further heterogeneity within each of the two main lymphocyte populations. Certain lymphoid cells are capable of mediating cytotoxicity against target cells coated with specific antibody. These cells are termed K cells, and although some evidence seems to suggest that they comprise a subpopulation of B lymphocytes, their nature and localization in lymphoid organ is not well known. This article will attempt to cover the methodologies involved in assays of T, B, and K cells. The methods to be discussed are used to detect the presence of different lymphoid cell populations; the markers used are not necessarily related to cell function.