Receptor tyrosine kinases of the ErbB family have been implicated in the onset/progression of a number of neoplasias. In these diseases, ErbB receptor expression may be accompanied by constitutive activation caused by molecular alterations, overexpression, or ligand binding. An important signaling route activated by these receptors that has been linked to the stimulation of cell proliferation is the Ras route. Here we have investigated the action of a mutant Ras form, H-RasN17, on the proliferation of the breast cancer epithelial cell line MCF7 cells. In these cells expression of RasN17 failed to affect serum or ErbB receptor-stimulated proliferation. Analysis of the action of RasN17 indicated that overexpression of this mutant form of Ras did not affect neuregulin or protein kinase C-induced activation of Erk1/2. In addition, RasN17 failed to prevent activation of endogenous N-Ras and H-Ras even though the levels of the latter were much lower than those of the RasN17 form. The failure of RasN17 to prevent endogenous Ras activation did not appear to be due to deficient processing or sorting of the mutated form. These data indicated that the action of RasN17 as a bona fide inhibitor of Ras depends on the cell type and requires detailed analysis of the biochemical and biological properties of RasN17, particularly with respect to the activation of endogenous Ras.