The oligomeric small heat-shock protein hsp27, also denoted hsp28, is constitutively expressed in several mammalian cells and displays a phosphorylation status that is related to cellular growth and differentiation. This protein is related to α-crystallin and has strong sequence similarity with an in vitro inhibitor of actin polymerization. Here, we have analyzed hsp27 phosphorylation, cellular localization and structural organization following serum stimulation of serum-starved HeLa cells. hsp27 is dephosphorylated in starved cells and quantitatively recovered in the form of small structures (<200 kDa) present in the soluble phase of the cytoplasm. Immediately after the addition of serum to starved cells, a rapid phosphorylation and complex changes in the intracellular distribution and structural organization of hsp27 are observed. Phosphorylation essentially occurs at the level of small hsp27 structures (<200 kDa) and is concomitant with the increased molecular mass (up to 700 kDa) of a fraction of this protein. Serum treatment also induced the detergent-sensitive association of another fraction of hsp27, still in the form of small and dephosphorylated structures, with cellular particulate fractions. Contrasting with these observations, hsp70 had the tendency to concentrate into nucleoli during serum starvation.