We developed a novel orthotopic mouse tumor model of renal cell carcinoma to collect and characterize cells spontaneously shed from SN12C (renal cell carcinoma) and SN12L1 (high metastatic variant of SN12C) tumors grown in kidneys of severe combined immunodeficient mice. Viability of the shed cell population was greater for SN12L1 tumors (25%) compared with SN12C tumors (11%, P < 0.05). Gene array analysis of 23 genes involved in metastasis showed that CD44, alpha3 integrin, and caveolin were down-regulated in the shed tumor cells compared with their primary counterparts, and blocking alpha3 integrin or CD44 function inhibited attachment and migration of both cell lines. These results suggest that cohesion of the cells within the primary tumor mediated by CD44 and alpha3 integrins hinders metastasis and that shedding is a passive process not necessarily mediated by cell migration in these tumors. Furthermore, resistance to apoptosis may enhance metastasis in the higher metastatic tumor.