1 Transfection of the pre-monomyelocytic U937 cell line with a plasmid coding for full-length annexin 1 (ANX1, 347 amino acid) leads to cell death by promoting apoptosis. In addition, overexpression of the N-terminal and the ®rst domain of the protein (144 amino acids, clone ANX1-S), which does not contain the Ca 2+ binding sites, gives susceptibility to cell apoptosis following activation by either 5 ng ml 71 tumour necrosis factor (TNF)-a or 1 ± 40 mg ml 71 etoposide. This was demonstrated by using the¯uorescent labelled annexin V, cell cycle and nuclear staining analyses. 2 Transfection with an empty plasmid (clone CMV) or with a plasmid carrying the cDNA antisense for ANX1 (clone ANX1-AS) did not alter U937 cells to the degree of apoptosis promoted by either stimulant. 3 Treatment of CMV U937 cells with TNF-a increased ANX1 mRNA and protein expression in a time-dependent manner, with maximal increases at 3 and 6 h, respectively. 4 Clone ANX1-S showed higher constitutive (more than 2 fold) and activated caspase-3 activity, associated with higher phospholipase A 2 (PLA 2 ) activity (in the region of +50 ± 100%), whereas expression of cytosolic PLA 2 Bax and Bcl-2 were similar in all cell clones, as determined by Western blotting. 5 In conclusion, this study demonstrates a complex regulatory role of cell apoptosis for ANX1, at least with regards to cells of the myelo-monocytic lineage.