Abstract: In an attempt to determine the influence of topical nitrogen mustard therapy upon epidermal Langerhans cells (LC), the quantitative, morphological and ultrastructural enzyme cytochemical studies on normal and nitrogen mustard treated guinea pig epidermal cells were performed respectively, by a new modified technique of ATPase LC staining for light and electron microscopy. With the computerized image analysis system we also made a quantitative morphological analysis on both normal and the treated epidermal cells. It was found that the cells after therapy showed a reversible decrease in number, vacuolation on cytoplasm and swelling of mitochondria. Some damaged LC were closely contacted with ATPase positive lymphocytes which was active in function. Thus it provides an experimental evidence that there are some correlations in function between topical nitrogen mustard application, epidermal LC and lymphocytes.