No consistent deleterious effects directly attributable to the nuclear polyhedrosis virus ofMamestra brassicae (L.),Autographa californica (Speyer) and the granulosis virus ofLaspeyresia pomonella (L.) were observed, when bone marrow cells of Chinese hamsters and NMRI mice were examined for chromosomal aberrations or sister chromatic exchanges. The animals were exposed to thein vivo andin vitro produced viruses acutely or chronicaly (90 resp. 97 days) by oral gavage or intraperitoneal injection. The treated animals neither showed signs of health disturbances nor abnormal behaviour nor changes in body weight in comparison to the controls.