Enzyme cytochemical (hydrolytic enzymes for cell differentiation), immunocytologic (B [bone marrow-derived] and T [thymus-derived] lymphocyte differentiation) and EM studies of skin infiltrates facilitate the proper diagnosis of myelomonocytic and lymphoreticular proliferations. With use of these methods, the original clinical diagnosis of malignant reticulosis of the skin was corrected to monocytic leukemia in a 65-yr-old women. Because primary involvement of the skin preceded monocytosis of the blood, the cutaneous infiltrates in this patient resulted from proliferation of tumor cells in the skin rather than from homing of the cells to, or settling of the cells in, the skin.