contamination of cell culture by another cell type - (Aug/15/2005 )

I have attempted to isolate endothelial cells from mouse lung. However, after several days in culture following an immunopurification step, my cell population has been taken over by another smaller cell type. It is not bacteria as the cells have coated the plate and there is nothing in suspension. I don't think it is mycoplasma either since I can see the cells by light microscopy. Doesn't look like a fungus either.

Does anyone have any ideas?

Thanks
Rachel

maybe a naive response, but are you sure it is not your original cells de-differentiating?