Cancer is considered an inflammatory condition where immune cells play an important role in progression and metastasis. Neutrophils may be pro- or antitumorigenic, depending on their phenotype or the number of infiltrating neutrophils in the tumor microenvironment. Massive infiltration of neutrophils in cancer tissue may elicit a cytotoxic effect, leading to tumor regression, whereas a S139 low-grade neutrophil gradient is tumor progressive. Chemokines, cytokines, and growth factors present in the tumor microenvironment, as well as cell-cell interactions mediated by integrins have shown to be determinant steps for cancer cells to break through the endothelial wall and establish metastatic niches. In this work we evaluated the role of lymphocyte functionassociated antigen 1 (LFA-1) integrin in neutrophils-mediated metastasis of estrogen receptor positive breast cancer cells (MCF-7) cells in a tumor xenograft model in zebrafish and in neutrophil infiltration in MCF-7 mammospheres. The metastatic capability of MCF-7 cells was evaluated in presence or absence of human neutrophils and with/without estradiol treatment. Two days old zebrafish embryos were injected into the perivitelline space with labeled MCF-7 cells and human neutrophils, an anti-human LFA-1 antibody (CD11a) was included. We show that estradiol treatment significantly increased the infiltration of neutrophils into MCF-7 mammospheres and this infiltration was significantly reduced by the presence of an anti-human CD11a antibody. Co-injection of MCF-7 cells with neutrophils significantly increased the migration of MCF-7 cells to distant sites in zebrafish and this effect was inhibited by using an anti-human CD11a antibody. We conclude that neutrophils affect the dissemination of breast cancer cells via LFA-1 integrin. Although estradiol increased the number of infiltrating neutrophils into mammospheres exposure to estradiol seemed to have minor effects on the dissemination in the zebrafish.