Human colon carcinoma cell lines were examined in a colonization assay using SCID-hu mice engrafted with human fetal lung (HFL) tissues. Cell lines SW620 and COLO 320DM, derived from metastatic tumors, colonized HFL grafts after i.v. injection into SCID-hu mice. Cell lines SW480 and T34 initiated from primary colon tumors were unable to colonize HFL grafts. The ability to colonize HFL grafts but not mouse lungs of SCID-hu-L mice correctly reflects the clinical origin of these human colon carcinoma cell lines. Expression of a number of cell adhesion molecules was examined on SW480, SW620 and in vivo selected highly aggressive variants of SW620. NCAM and integrin alpha 3 expressed on the surface of SW480 cells were lost from metastatic cells, while carbohydrate ligands sialyl Lewis x and a, previously shown to be upregulated in metastatic colorectal tumors, were expressed at higher levels on colonizing cells. Unlike SW480, SW620 and its in vivo selected variants expressed RNA for calcium binding protein calbindin-D28K, a neuroendocrine marker. Acquisition of neuroendocrine features might be of potential importance in the development of the metastatic phenotype.