Biochemical and genetic strategies have implied that abberant signaling in the extracellular signal-regulated kinase (ERK)/mitogen-activated protein (MAP) kinase pathway contributes significantly to transformed phenotypes. Using PD98059, an inhibitor of the ERK-kinase MEK1, we have here assessed the effects of ERK inhibition on the pattern of protein expression in the metastatic human breast cancer cell line MDA-MB-231. At a concentration of inhibitor which did not significantly affect cell growth, PD98059 induced large changes in the expression of MDA-MB-231 polypeptides. The majority of these changes were due to decreased expression of low-abundant proteins. Decreases of more abundant proteins such as glutathione-S-transferase pi, hsp80 and hsp100 were also recorded. The levels of a few proteins increased, among them cytokeratin 8. We conclude that PD98059 treatment of MDA-MB-231 cells induces large changes in protein expression.