The presence of T lymphocytes in the situ in cutaneous infiltrates of contact dermatitis and lichen planus has been demonstrated by an immunoelectronmicroscope technique. A specific anti-human T cell antiserum labelled with peroxidase-conjugated anti-immunoglobulin serum was used to reveal the specific human T lymphocyte antigen (HTLA) which was visualized ultrastructurally. This technique shows that labelling is limited to the surface membrane and allows study of the in situ interrelationship of T cells.