Previous studies in our laboratory using immunoelectron microscopy have shown that anti-T6 monoclonal antibody reacts with all epidermal Langerhans cells in normal skin. Comparison of the number of T6-positive (+) epidermal cells with Ia (HLA-DR) (+) cells, as defined by the monoclonal antibodies, anti-I1 and anti-I2, disclosed that these latter markers significantly underestimated Langerhans cell and indeterminate cell numbers (p less than 0.01 and p less than 0.001, respectively) when employed in a sensitive 4-step immunoperoxidase procedure. Thus, it appears that all epidermal Langerhans cells and indeterminate cells are not Ia-positive as defined in this system and that Ia(+)/T6(+) and Ia(-)/T6(+) subsets exist. These subsets may be analogous to the Ia(+) and IA(-) subsets of macrophages, in which the former are responsible for antigen interaction with T cells.