We report here that the mere fact of changing culture medium for fresh medium induced in several cell lines the expression of stress-activated genes including protein kinases p38, JNK and ERK1/2 and the transcription factor C/EBPβ. As a consequence, p8, a gene induced by stress in several tissues, was strongly up-regulated. Induction did not occur after change for cell-conditioned medium. Induction was however transient, with a peak at 60 min for p38, at 15 – 30 min for JNK and at 15 min for ERK1/2, at 2 – 3 hours for C/EBPβ and at 4 – 6 hours for p8. Repression of the induction was due to the secretion of thermolabile molecule(s) that progressively conditioned the medium. As low as 25% of conditioned medium added to fresh culture medium was sufficient to abolish the stress response. Taken together, our data indicate that the renewal of culture medium induces a transient cellular stress that may be a source of artifacts in experiments performed shortly after a change of culture medium.