HSP27 form oligomeric structures up to 800 Kda. In cultured cells, the equilibrium between small and large oligomers shifted towards smaller oligomers when phosphorylated on serine residues. To further explore HSP27 structural organization and its repercussion in HSP27 antiapoptotic and tumorigenic properties, we transfected colon cancer REG cells with wild type HSP27 and two mutants in which the phosphorylatable serine residues have been replaced by alanine (to mimic the non phosphorylated protein) or aspartate (to mimic the phosphorylated protein). In growing cells, wild type and alanine mutant formed small and large oligomers and demonstrated antiapoptotic activity while aspartate mutant only formed small multimers and had no antiapoptotic activity. In a cell-free system, only large oligomeric structures interfered with cytochrome c-induced caspase activation, thereby inhibiting apoptosis. The inability of the aspartate mutant to form large oligomers and to protect tumor cells from apoptosis was overcome by growing the cells in vivo, either in syngeneic animals or nude mice. These observations were reproduced by culturing the cells at confluence in vitro. In conclusion (1) large oligomers are the structural organization of HSP27 required for its antiapoptotic activity and (2) cell-cell contacts induce the formation of large oligomers, whatever the status of phosphorylatable serines, thereby increasing cell tumorigenicity.