This report defines the influence of field soft X-ray irradiation on the integrity of epidermal Langerhans cells (LC) in the guinea pig and mouse systems. Male albino (Rockefeller strain) and piebald F 1 (2 X 13 strain) guinea pigs as well as C3H mice (H-2Kk) were exposed to one single shot of different dosages of soft X-rays [80, 1,200, 1,600, and 3,200 R; 30 kV, 0.5 mm aluminium filter, FSD (focus-skin distance) 12 cm]. 1 and 4 weeks after exposure, skin specimens were taken from the irradiated skin. The demonstration and evaluation of LC was performed basing on their expression of specific histochemical (ATPase) and functional immunologic markers (Ia antigens). Soft X-irradiation had pronounced effects on number and structure of ATPase- and Ia-antigen-positive cells. In the mouse system, 1 week after exposure to 800 and 1,200 R ATPase-positive cells and, in a more or less parellel manner, Ia-antigen-positive cells () were reduced to 74% (74%) and 70% (68%), respectively, and 4 weeks after exposure to 49% (47%) and 43% (40%), respectively. In the guinea pig system one single shot of 1,600 or 3,200 R produced, respectively, a 35% or a 38% reduction of ATPase-positive cells. Prolonged survival did not result in a further depletion of ATPase-positive cells.