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Cell-free fluid from several ascites promoting tumors inhibits lymphocyte trapping. Lactic dehydrogenase virus (LDV), a common passenger virus in many mouse tumors, was found to be a trapping inhibitor component in these fluids. Procedures used to eliminate infective LDV, such as dilution, passage of the tumor through irradiated rats, ether fractionation, and ultraviolet (UV) irradiation abolished the trapping inhibitory capacity of the fluids. LDV, dissociated from tumors, was inhibitory. Lymph nodes in mice with acute, but not chronic, LDV infections were inhibited from trapping. LDV does not appear to inhibit the capacity of circulating cells to be trapped, and as measured by mitogen responsiveness, the virus does not directly interfere with T-cell function. LDV may inhibit trapping by indirectly affecting the T cell or directly affecting the macrophage in which it replicates. The known characteristics of LDV infection may explain a number of reported immunosuppressive attributes of tumor- associated ascitic fluids.