Although proto-oncogene expression has been shown to correlate with clinical outcome in breast carcinoma, an experimental model has not been proposed to study this phenomenon in vivo. In addition, the ability to modulate this proto-oncogene in vivo to correlate with phenotypic behavior has not been determined. Utilizing an intraperitoneal model for metastatic spread with BT20 human breast carcinoma cells, clonally expanded cells expressing five fold higher c-fms protein were compared with parent BT20 cells as well as an underexpressing clone using intrasplenic injection following left flank cut-down in female nude and Severe combined immunodeficient (SCID) mice. Athymic BALB/c nude and SCID animals were observed for clinical evidence of tumorigenicity with necropsy performed at either 50 or 80 days unless compromised earlier. Immunohistochemistry (IHC) of the harvested tumors was performed to correlate c-fms expression from its original in vitro culture to the in vivo model. At day 50, differences in primary tumor take and spread to the pelvis were already evident favoring the c-fms over-expression group with IHC of these tumors revealing significantly higher intensity of staining for c-fms, (mean H score of 205 vs. 43 in the over-expression and parent groups, respectively). At day 80, tumor take and spread was comparable; however, tumor size in the over-expression group was significantly larger than the parent and under-expressing group in both the BALB/c and SCID experiments. Modulation of c-fms proto-oncogene expression was also achieved using the anti-glucocorticoid, RU-486, via oral administration to SCID mice with subsequent correlation to IHC staining. This model thus provides tumors of significant size and organ diversity which retain their phenotype early in tumorigenesis allowing an early endpoint to assess efficacy of novel treatments.