Monocytes from patients with head and neck cancer produce excessive PGE2 which seems to be associated with decreased lymphoproliferation in vitro. This may be n pathological exaggeration of a normal homeostatic mechanism. It is speculated that a stimulus for excessive PGE2 production could be immune complexes. Although PGE2 suppresses many important immune processes relevant for neoplasia, it can also have complex and diverse effects on tumor cells. Caution should be exercised with human trials of prostaglandin synthctase inhibitors such as indomethacin.