The effects of reconstituted basement membrane (Matrigel) on in vivo survival and growth of several murine tumors were studied. Survival of tumor cells was enhanced in all experiments which resulted in increased incidence and/or in increased tumor mass. While basement membrane enhanced the in vivo growth of B16F6 melanoma cells, survival of these mice was prolonged. Basement membrane increased the incidence but reduced the growth of Ehrlich ascites tumor. Walker‐256 hypercalcemic breast carcinosarcoma growth was enhanced and glandular‐like structures were observed when grown on Matrigel. The results indicate that the enhanced survival of tumor cells in the presence of basement membrane is not unequivocally linked with increased malignancy.