T cells from mouse spleen and lymph node do not express the cell-surface glycoproteins encoded for by MHC class II genes, whereas most non-T cells in these organs do (i.e., B cells and accessory cells). This unique feature of T cells makes it possible to enrich for them using cytotoxic anti-class II monoclonal antibodies and activating complement. Because there are two isotypes of MHC class II genes (A and E genes, located adjacent to each other in the MHC region originally called the I region), it is necessary to identify which of these two genes are expressed in the mouse strain used. This unit lists suitable antibody-producing hybridomas available from ATCC, with antigen specificities (I-A vs. I-E) and corresponding mouse strains (H-2 strains a, b, k, etc., indicated as superscripts). Other antibodies can be tested in pilot experiments using the procedure outlined. This unit describes T cell enrichment using cytotoxic antibodies, and unit 3.4 describes the depletion of T cells and their subpopulations using the same approach. In the latter unit, T cell surface markers (Thy-1, CD4, and CD8) are targeted by the cytotoxic antibodies.