An electron microscope study of 5 human biopsy specimens from suspected mesotheliomas showed that the tumor cell fine structure was usually similar to that of mesotheliomas produced in laboratory animals by the intraperitoneal injection of asbestos dust. In most instances, the tumor cells had a well-developed granular endoplasmic reticulum, and the cell cytoplasm often contained bundles of microfilaments. In some, these microfilaments made up a large part of the cell volume. Glycogen was found in all the neoplasms examined, and some had large amounts. Lipid granules, seen in the cytoplasm of many tumor cells, had a characteristic crenated surface. The cell surface membranes were characteristically modified to form numerous surface processes of variable type found usually around most of the cell circumference. The processes ranged from irregularly shaped mi crovilli to globular projections like miniature pseudopodia, and often these 2 forms were intermingled on the same cell. One biopsy specimen that did not conform to this general pattern was proved at autopsy to be a bronchial carcinoma with secondary spread into the pleural cavity. This indicated that electron microscope examination may be extremely useful as an aid to diagnosis when only small biopsy specimens are available.