The localisation of subpopulations of T lymphocytes as defined by the monoclonal antibodies W3/25 and MRC OX 8 was determined by an indirect immunoperoxidase technique on cryostat sections of rat lymphoid tissues. Both subpopulations were present throughout T-dependent areas with only scattered T cells, largely W3/25 positive, in the B areas. The W3/25 antigen was also found on macrophages in the medulla of lymph nodes and in the peritoneal cavity and liver.