Trastuzumab or Herceptin is an antibody directed against Her-2. Her-2 is a growth factor receptor which is present on the tumors of 25% of patients with breast cancer. The addition of trastuzumab to chemotherapy has been shown in a randomized clinical trial to increase the response rate to chemotherapy, the duration of response to chemotherapy, and to improve the duration of survival of patients with metastatic breast cancer. The anticancer mechanism of action of trastuzumab is unknown, but it is possible that trastuzumab acts by promoting antibody-dependent cell mediated cytotoxicity (ADCC), or direct killing of cancer cells by immune cells, triggered by antibodies bound to the surface of the cancer cell. G-CSF is a drug which is a growth factor for certain types of immune cells. G-CSF has two favorable effects on ADCC. G-CSF increases the pool of circulating cancer-killing immune cells, and G-CSF increases the strength of binding of cancer-killing immune cells to a specific part of the antibody. Therefore, priming with G-CSF significantly increases the efficiency of ADCC, and four days of treatment with G-CSF has been shown to optimize ADCC in some studies. Recent data from the investigators' laboratory indicates that chemotherapy can augment ADCC directed against tumor cells.