Different biological materials were tested for murine viral contamination by using the mouse/rat antibody production test. Of 297 tumors examined, 75 (25.3%) were contaminated. Considerable differences in the contamination rate became evident when transplantable tumors from in vitro and from in vivo passages were compared. Of 186 tumors that had been propagated in animals, 36.6% were positive, whereas only 7 of 111 (6.3%) tumors propagated in vitro were contaminated. The highest rate of contamination was detected in mouse tumors. Testing of 135 specimens of mouse origin revealed 46.7% were contaminated, and 57 (70.4%) of 81 samples propagated in mice were positive for murine viruses. Moreover, 6.7% of 90 human tumors that had been passaged in athymic nude mice and 3.5% of 57 rat tumors were positive. Lymphocytic choriomeningitis virus was detected in 4 of 14 hamster tumors. The most frequent contaminant was lactic dehydrogenase elevating virus followed by reovirus 3, lymphocytic choriomeningitis virus, minute virus of mice, mouse hepatitis virus, rat coronaviruses, Kilham rat virus, and Mycoplasma pulmonis. Contamination with reovirus 3 and minute virus of mice was found in 4 (3.7%) of 109 cell lines tested, and 2 of 60 monoclonal antibody preparations or hybridoma cells contained lactic dehydrogenase virus. Contamination with two pathogens was detected in four mouse tumors and in one cell line.