A new cell line from a neoplastic ascites was established. This strain, designated PAT-206, was characterized by plastic adherence and a high proliferative potential without any specific growth factor requirement. Karyotype analysis showed that the line was of human chromosomal constitution and aneuploid. Surface marker analysis showed that CD45, CD33 and CD15 were positive. In addition, the presence of human cytokeratins was detected by cytoplasmic immunofluorescence. Interestingly, the cell line did not express major hystocompatibility complex (MHC) class I and II, and was more sensitive than the 'classic' K562 cell line, to killing mediated by fresh uncultured peripheral blood lymphocytes. Following differentiation with interferon-gamma, the cell line expressed MHC class I antigens and resulted resistant to natural killing mediated lysis. This novel NK cell target seems to be suitable for further studies on NK cell specificities.