Malignant histiocytosis with cutaneous involvement occurred in a 27-year-old man. There were generalized weakness, abdominal pain, lymphadenopathy, fever and firm, papulonodular cutaneous infiltrations. The tumour cells were identified as malignant histiocytes by specially adapted enzyme-cytochemical and immunocytological methods on cryostat sections and single-cell suspension from the cutaneous infiltrates. The malignant histiocytes are characterized by diffuse activity of esterases and acid phosphatase. In addition, tests in single-cell suspension make it possible to differentiate the cells further according to cytomorphological and enzyme-cytochemical criteria.