B lymphocytes play a role in inhibiting the immune response against certain tumors, but the underlying mechanisms are poorly understood. EMT-6 mammary tumors grow well in wild-type (WT) mice but show reduced growth in B-cell-deficient μ−/− BALB/c mice (BCDM). WT mice demonstrate extensive B-cell infiltration into the tumor bed, reduced CD8+ T cell and CD49+ NK cell infiltration, and markedly reduced cytolytic T-cell response relative to BCDM. Expression of LAP/TGF-β1, CD80, CD86 and PD-L1 is significantly increased in tumor-infiltrating B cells (TIL-B) relative to splenic B cells. LAP/TGF-β1 expression on TIL-B progressively increased from 5.4±1.7% on day 8 to 43.1±6.1% by day 21 post tumor implantation. Co-culture of EMT-6 tumor cells with Naive-B cells ex vivo generated B cells (EMT6-B) with a similar immunophenotype to TIL-B. Purified TIL-B, or in-vitro-generated EMT6-B suppressed CD4+, CD8+ and CD4+CD25− T-cell proliferation, and Th1 cytokine secretion, and also suppressed purified NK-cell proliferation in response to IL-15, compared to naive splenic B cells. Acquired B regulatory function required direct tumor cell: B-cell contact, and was partially reversed by antibody to TGF-β or PD-L1, leading to tumor rejection in vivo. B-cell acquisition of a suppressive phenotype following tumor infiltration may result in profound inhibition of T-cell anti-tumor responses.