CD44 is a cell surface glycoprotein that functions as an adhesion molecule for the extracellular matrix, and is alternatively spliced to form mRNA variants. It has been demonstrated that expression of CD44 variants may be associated with many invasiveness cancers, including breast cancer. We have shown that patterns of CD44 expression are different between ER$-$ (highly invasive) and ER+ (less invasive) breast cancer cells. In ER$-$ breast cancer cell lines, the CD44E (epithelial) variant, containing the alternately spliced exons v8 to v10, is reduced when compared to CD44H (standard form). As well, there is a decreased incorporation of v7 and v10 containing variants in ER$-$ cells. This difference is partly conserved in ER$-$ and ER+ tumours. Based on these studies, our aim is to determine if the tumor cell microenvironment influences CD44 splicing patterns, and whether the difference in CD44 expression contributes to increased invasiveness of ER$-$ cell lines. We studied the effects of cell density and cell substrates on both ER+ and ER$-$ cells to determine the effects of the microenvironment on CD44 expression and splicing. (Abstract shortened by UMI.)