In vivo depletion of CD4- and CD8-specific T cells is a means of studying the role of these subpopulations in the initiation and effector phases of particular in vivo immune responses. In this unit, a protocol is provided for harvesting anti-CD4 or anti-CD8 monoclonal antibody- producing ascites fluid or tissue culture supernatant from rat or mouse T cell hybridomas. The antibody (preferably IgG) is then purified and injected intraperitoneally into adult mice. Depletion of the appropriate subset of T cells is verified by flow cytometry analysis of lymph node and spleen cell suspensions in pilot experiments. Once conditions have been established, depleted mice can be used to study the impact of T cell subsets on in vivo immune responses. The depleted condition is maintained by repeated injections of the monoclonal antibody.