Several similarities exist between Langerhans cells (LC), a subpopulation of the mammalian epidermis, and cells of the monocyte-macrophage-histiocyte series, including the expression of common surface marker characteristics (Fc-IgG and C3 receptors). In further analogy, we asked the question of whether these cells also bear the human equivalent of murine and guinea pig Ia antigens, i.e., B cell alloantigens. Since LC constitute only 2 to 4% of isolated epidermal cells, cytotoxicity techniques did not seem feasible for this purpose. We therefore elaborated an indirect immunofluorescence (IF) technique for the detection of B cell alloantigens, using fluorescein-isothiocyanate-labeled staphylococcal protein A (FITC-SPA) as indicator reagent. Alloantisera were obtained from multiparous women of a highly inbred population. In simultaneous experiments we tested purified B cells from seven normal donors against a panel of alloantisera by a microcytotoxicity test and by IF. Data obtained showed that the alloantigens are selectively expressed on SIg-positive, but not on SIg-negative lymphocytes. Furthermore, there was an excellent agreement between both techniques (correlation coefficient: X2 = 31.09, p <0.001). Isolated epidermal cells from those individuals whose B cells were tested for the expression of alloantigens were subjected to the IF procedure described above. None of the antisera which gave negative reactions on purified B cells of the respective individual reacted with its epidermal cells. By contrast, several of the antisera stained a small percentage of epidermal cells of those individuals whose B cells reacted with the very same antisera. By a simultaneously performed rosette assay with IgG-antibody-coated bovine red blood cells, it could be shown that the fluorescent and rosetted cell populations were identical, which means that—by IF techniques—LC are the only epidermal cells expressing B cell alloantigens.