Previous studies in our lab have shown that mice with a deleted Signal Transducer and Activator of Transcription-6 gene (Stat6-/- mice) have heightened immunity to a highly malignant and spontaneously metastatic BALB/c-derived 4T1 mammary carcinoma. We hypothesized that Stat6-/- mice might also be resistant to mammary tumors arising from the activated rat HER-2/neu oncogene (Stat6-/- x NeuT mice). Preliminary studies show that tumor onset in Stat6-/- x NeuT mice is somewhat delayed when compared with the BALB/c x NeuT control mice. This study was undertaken to confirm that Stat6-/- mice are in fact more resistant to the HER-2/neu tumor by monitoring tumor onset in greater detail, and also to elucidate the mechanism underlying tumor resistance by looking at cytokine production, and differences in lymphocytes in Stat6-/- x NeuT mice when compared with control mice. Stat6-/- BALB/c females were mated with Stat6-/- x NeuT males carrying the HER-2/neu oncogene and the progeny was screened for the transgene by PCR. The immune status of the mice was looked at following challenge with the 4T1 mammary carcinoma. Splenocytes, and in some cases the draining lymph node lymphocytes, were analyzed by flow cytometry for the percentages of cells with surface markers CD4 (helper T-cells), CD8 (cytotoxic T-cells), and B220 (B-cells). The lymphocytes were also stimulated in vitro with 4T1/B7 cells and ELISA assays were used to measure the amount of cytokines produced as a measure of T-cell activation. The cytokines analyzed were interleukin-2 (IL-2, a T-cell growth factor), interleukin-4 (IL-4, made by TH-2), and interferon-γ (INF-γ, made by TH-1). Although no consistent differences were seen between NeuT, Stat6-/-, Stat6-/- x NeuT, and BALB/c mice, unusually large cells were found in the spleens of NeuT and Stat6-/- x NeuT mice. Future experiments will be aimed at identifying this unusual population of cells.