The inflammatory cell infiltrates in scalp skin of 35 patients, 20 with alopecia areata (AA), 7 with totalis, and 8 with universalis were characterized with the ANAE (alpha-naphthylacetate esterase) marker, monoclonal antibodies, and electron microscopy. As demonstrated by the ANAE staining, no clear difference in the main lymphocyte subclasses (T and B cells) or macrophages was seen between the different types of alopecia or as compared to control patients' scalp skin. However, T lymphocytes and macrophages were seen most frequently and in greater numbers perivascularly and infiltrating the hair bulb in those cases of AA where active hair loss took place. Using monoclonal OKT (OKT-3, -4, and -8) antibodies and the avidin-biotin immunoperoxidase method on frozen sections, a concentration of OKT-8 reactive cells (suppressor/cytotoxic T cells) was seen peribulbarly and invading the hair infundibulum. The cells affecting the hair infundibulum were further studied by electron microscopy. They could be classified into three main types: small lymphocytes (60%), macrophages (30%) and cells closely resembling large granular lymphocytes (LGL) (10%). LGL have previously been considered to be human natural killer (HNK) cells. Thus the hair follicle seems to be the target for the cellular immune response in alopecia. Whether HNK cells participate in the destruction of hair bulbs remains to be investigated.