In 11 patients with untreated alopecia areata in the progressive stage of the disease, an in situ analysis of the inflammatory infiltrate of the hair bulbs was performed by means of different monoclonal antibodies. Most of the peribulbar cells reacted with the pan T-cell antibodies OKT 3 and Lyt 3. Staining for T-cell subsets revealed that the proportion of OKT 4+ cells was about fourfold higher than that of OKT 8+ cells. Almost all of the T cells were OKIa1+, indicating that they were in an activated state. In four of the 11 cases, both subsets of T lymphocytes were also found to infiltrate the hair matrix itself. These results would appear to be consistent with the assumption that alopecia areata is caused by a T cell mediated autoimmune mechanism.