In a previous paper, we have shown that the c-erbB-2-encoded protein p185 erbB2 is localized on the brush border of the proximal tubule kidney ceils. In invasive duct cell carcinomas, the labeling was most obvious on the villous plasma membrane protrusions. From these observations the hypothesis was raised that p185 erbB2 could play a role in motility. To test this hypothesis, we quantified its distribution on the microvilli and plasma membrane protrusions and on the straight parts of the cell membrane after immunoelectron microscopy. These f'mdings were compared with the localization on p185 erbB2 overexpressing SK-BR-3 human breast cancer cells before and after stimulation of motility by treatment with conditioned medium from COLO-16 cells (CM), which is also able to induce chemotaxis of these cells in a Boyden chamber assay. In the invasive duct ceil carcinomas, the number of gold particles was nine times higher at the plasma membrane protrusions than at the straight parts of the cell membrane. In untreated SK-BR-3 cells, p185 erbB2 was similarly concentrated on the membrane of small microvilli and plasma membrane protrusions. Treatment of SK-BR-3 cells with CM leads to ceil spreading, enlargement of the microvilli, formation of pseudopodia and chemotaxis. Aggregation of p185 erbn2 at the plasma membrane protrusions and pseudopodia is observed on immunofluorescence light microscopy. The concentration of p185 erbB2 is several times higher on these membrane extensions than on the straight parts after immunogold labeling. It is concluded that p185 erbn2 is localized on cell organelles involved in motility, and it is suggested that the molecule plays a role in cell movement, providing the capacity of tumor cells to spread and metastasize.