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The function of the immune system during tumor growth is very controversial. Although there were great hopes that a proper stimulation of immunity would erradicate tumor cell proliferation this objective has still not been attained. We review and analyze some of the immunologic studies performed in our laboratory using spontaneous murine mammary adenocarcinomas. In early stages of tumor growth our studies showed a specific immune response in vivo and in vitro by delayed type hypersensitivity tests, and syngeneic lymphocyte induced angiogenesis assay. These activated lymphocyte responses were not involved in mechanisms effective for tumor rejection. The lymphocyte activation vanished with tumor growth. Soluble factors shed from splenocytes of tumor bearing hosts and factors secreted by tumor cells enhanced tumor and metastatic growth. Other cell populations, such as mastocytes and neutrophils were also altered during tumor growth. We conclude that soluble factors released by tumor cells induce the production of suppressor cells or factors that stimulate tumor and metastatic growth.