The transmission (TEM) and scanning electron microscopic (SEM) features of cancer cells and benign cells from human effusions were described. Both in the TEM and the SEM the following cell types were indentified; mesothelial cells, histiocytes, lumphocytes, eosinophilic leukocytes, neutrophilic leukocytes, plasma cells, erthrocytes and cancer cells. The ultrastructure of malignant cells originating from cancers most commonly metastisizing to the serous body cavities was described in detail. Both the TEM and the SEM clearly demonstrated fundamental differences between the surfaces of the cancer cells and benign cells in body cavity fluids: cancer cells were covered by numerous microvilli, while non-neoplastic cells were not. The approach described in this paper can be used for purposes of cytologic diagnosis in difficult preselected cases and may also have some important theoretical implications.