The author has studied the behaviour of cells from human ascitic fluid in long-term culture (5–6 months). Three cellular types are described with different morphological features, namely the cellular shape, the fashion in which the cell spreads on the glass, the nucleo-cytoplasmic ratio, the chromatin appearance and the abundance of mitochondria. The three cellular types can phagocytose, but each one in a different way. The first one phagocytoses exclusively erythrocytes ‘by contact’ without emission of pseudopods; the second one phagocytoses degenerating nucleated cells in the same way as the first; the third type phagocytoses degenerating nucleated cells by emission of long pseudopods. The origin of these three cellular types is discussed; it is felt that they are transformed mesothelial cells. According to this study, it cannot be excluded, especially for the second and the third type, that they are histiocytes coming from serous membranes. The life in vitro of the three cellular types is depending upon the composition of nourishing medium. Cells can divide by mitosis only during the first 10–15 days of culture (mitotic index 0.1–3.0‰). Nuclear amitosis, nucleolus expulsion into cytoplasm and cytoplasmatic DNA synthesis can be observed in he