Seven of 27 students and staff oa a university laboratory became ill with lymphocytic choriomeningitis (LCM) following exposure over an 18-month period to hamsters implanted with tumors. The clinical symptoms were compatible with LCM infection and all the patients' sera had fluorescent antibody to LCM virus; 5 of them also had complement-fixing (CF) antibodies. Complement-fixing antibodies were found in 9 of 9 hamsters with tumors and 16 of 31 without tumors (excluding 6 hamsters with anticomplementary sera) and in 8 of 14 rabbits, but in none of 49 rats. Lymphocytic choriomeningitis virus was isolated from 4 pools of liver and spleen, 2 each from hamsters with and without tumors, and from a frozen stored sample of the tumor cells. Interlaboratory transmission may have occurred via infected tumor cells. Control was achieved by destroying all animals in the project room as well as all hamsters in the building. The possibility that many tumor cell lines carried in hamsters may be infected with LCM virus has important implications for laboratory safety as well as for the validity of the research results obtained, inasmuch as the virus enhances the growth of some tumors and retards others.