Mouse transplantable tumors showed the highest frequency of contamination, whereas tumors of other species showed much lower frequencies (Table IV) . Contamination with reovirus 3 and MVM was found in 4 (3.7%) of 109 cell lines tested, and in 2 of 60 monoclonal antibody bulk preparations. With respect to LCMV, Bhatt et al. (1986) reported its isolation from transplantable tumor cell lines. The testing of tumor cell lines revealed that 16 out of 55 in vivo tumor samples and one out of eight in vitro samples were positive. A similar situation was found in a New Jersey research institute, where human cell lines and tumor cell lines were passaged via nude mice for the development of monoclonal immunodiagnostics and immunotherapeutic agents. This LCMV contamination led to the outbreak of laboratory-acquired human LCMV infection (Mahy et al., 1991) . LCMV contaminated hamster tumor cell lines have also been responsible for an outbreak of infections occurred in medical center personal at the University of Rochester (Hay, 1991) .