Transmission of viral infection by tumour lines or other biological materials may have confounding effects on research Many research organizations require screening for viral agents of all cell lines, tumours, sera and other biologicals before implantation or inoculation into animal models. Screening for viral contamination is done by the mouse antibody production （MAP） test, by cell culture, or alternatively by direct detection of the viral agents by polymerase chain reaction （PCR）. The description of procedures for sanitation of infected cell lines or tumours is sparse. The present report describes the procedures used for sanitation of three transplantable murine tumour lines, which were transplanted in vivo in a mouse hepatitis virus （MHV）-infected colony of mice at the Department of Experimental Clinical Oncology （DECO）. The tumours were frozen and serially transplanted three times in a quarantine colony of syngenic mice.