The distribution of ferritin in 36 autopsy cases of malignant histiocytosis was investigated by immunocytochemical staining, together with the detection of alpha 1-antichymotrypsin, alpha 1-antitrypsin, lysozyme, S-100 protein, and ricinus communis agglutinin in the consecutive sections. The results showed that ferritin-positive tumor cells were present in every case. The quantity of cellular ferritin in well-differentiated histiocytes was higher than that in atypical histiocytes. Double labeling showed that ferritin and alpha 1-antichymotrypsin might be located either in one tumor cell or in separate cells. Our data suggest that ferritin may be a tumor associated antigen in malignant histiocytosis, playing a regulatory role for tumor cell differentiation.