We have used human mammary cells of the MCF-7 strain, which constitutively express high levels of the small heat shock protein HSP27 and we have compared the changes in the phosphorylation status of this protein together with changes in cell growth and/or morphology induced by the action of one of the following agents: (1) TPA (12-O-tetradecanoylphorbol-13-acetate), known as a differentiation inducer in MCF-7 cells; (2) OH-TAM (hydroxytamoxifen), which exerts a cytostatic and cytotoxic action; or (3) TNF alpha (tumour necrosis factor), which induces apoptotic cell death in this cell line. Our data show that TPA and TNF stimulate an immediate and massive phosphorylation of HSP27, whereas OH-TAM affect the phosphorylation status of the protein only after a 3 day delay. In the case of TPA, high levels of HSP27 phosphorylation were maintained for at least 4 days, along with growth inhibition and acquisition by the cells of a secretory phenotype. TPA and OH-TAM exerted similar immediated effects on cell growth, despite the different time course of their action on HSP27 phosphorylation. This excludes the possibility that the latter is a necessary consequence of, or an absolute requisite to, growth inhibition. With OH-TAM and TNF the increase in HSP27 phosphorylation was concomitant with the appearance of apoptosis, not observed with TPA. This indicates that increased phosphorylation of HSP27 is not specifically associated with the triggering or the execution of apoptosis in these cells. Altogether, our data support the concept that phosphorylated HSP27 is involved (and might then be rate limiting in some instances) in the execution of vital cell programmes (including resistance to stress, proliferation and differentiation), as well as in that of cell death. This is consistent with its role in actin polymerization and its position downstream of the p38/RK-type MAPkinase, itself a point of convergence for diverse signal transduction pathways.