Two models of in vitro tumor growth will be presented. (1) Transforming growth factor TGF is known to have properties of both tumor suppressor and tumor promoter. While it inhibits cell proliferation, it also increases cell motility. A mathematical model quantifies the growth of MCF10A/HER2 cell cultures in vitro under exposure to TGF. The model supports the hypothesis that TGF increases the tendency of cells and cell clusters to move randomly, while simultaneously diminishing cell proliferation. (2) P-glycoprotein (P-gp) is a protein over-expressed in cancer cells that causes multi-drug resistance to cancer therapy. Recent experimental evidence demonstrates that P-gp is transferred directly cell-to-cell in in vitro tumor cell lines. A mathematical model quantifies the transfer process of P-gp in in vitro cultures of MCF-7 human breast adenocarcinoma cells. The model supports the hypothesis that P-gp is transferred directly cell-to-cell and provides a framework for optimizing chemotherapy regimens.