Overexpression of the crbB-2 gene-encoded p185(c-erbB-2) is correlated with early onset of metastasis in breast cancer patients. Furthermore, the detection of blood-borne epithelium-derived clustered cells expressing p185(c-erbB-2) was related to advanced stages in breast cancer. To further elucidate the receptor's function in the metastatic process of human breast cancers, we analyzed disaggregated cells and cell clusters from freshly dissected breast cancer tissues. We studied whether their capability of extravasation is correlated with their expression of c-erbB-2. A model for the venular wall was constructed by growing human umbilical vein endothelial cells (HUVECs) on porous membranes coated with basement membrane extracellular matrix. In four control breast cancer cell lines (SK-BR-3, MCF-7, MDA-MB-468, and MDA-MB-468, the latter transfected with a full-length c-erbB-2 cDNA vector) producing different levels of the c-erbB-2 receptor, the expression level correlated positively with the invasiveness of the cells. The invasive, predominantly clustered cells from 14 of 23 tumors were positively stained for p185(c-erbB-2) by immunocytochemistry. Furthermore, we show that the invasive cell populations express the metastasis-associated proteins matrix metalloproteinase MMP-2, CD44, and integrins alpha(v)beta3 and alpha6. In this first study on the behavior of cells and cell clusters from disaggregated operated cancers in an extravasation model, we could demonstrate the presence of c-erbB-2-expressing cell subpopulations within the individual breast cancers that are presumably of high metastatic potential.