The nature of hyaline bodies (HB) in Kaposi's sarcoma (KS) has been investigated by electron microscopy (EM) and immunohistochemical methods. Paraffin sections from 45 cases of KS selected on the basis of their high content of HB were challenged with antisera against factor VIIIR:Ag, carcinoembryonic antigen (CEA), alpha-fetoprotein (AFP), alpha 1-antitrypsin (A1AT), fibrinogen, hemoglobin, alpha-actin and lysozyme. HB showed positivity for all the antibodies except for the last two. By EM, HB showed features consistent with red blood cell, fibrin and platelet phagocytosis. Therefore, HB in KS are considered to be the expression of an indiscriminate process of phagocytosis which involves not only erythrocytes and platelets, but also other substances such as fibrinogen, factor VIIIR:Ag, A1AT, CEA and AFP.