The tumor-associated transplantation antigens (TATA) of crude membrane extracts from SV40-transformed BALB/3T3 tumor cells lytically infected with influenza virus were markedly more immunogenic than were extracts from uninfected cells measured either by the ability to induce heightened resistance to tumorgraft challenge or by heightened lymphocyte-mediated cytotoxicity against tumor cells in vitro. When intact tumor cells (as opposed to membrane extracts) were productively infected with Moloney sarcoma virus, they were made so immunogenic that they would only grow in X-irradiated syngeneic animals. Yet crude membrane extracts from the Moloney sarcoma virus-infected tumor cells showed no increase in TATA activity analogous to that seen after infection with influenza virus. Thus, influenza virus augmentation of tumor membrane TATA may operate by a different mechanism than does the oncornavirus augmentation of intact tumor cell TATA reported by others. It appears that Moloney sarcoma virus and possibly other oncornaviruses cannot be used to augment the TATA activity of tumor cell membranes in the same way that other surface-budding viruses can.