A new locus of wild house mice tolerantly infected with the virus of lymphocytic choriomeningitis (LCM) has been identified in the United Kingdom. Evidence is presented which indicates that these mice were the source of infection in a laboratory mouse breeding colony, the mode of transmission probably being bites on tails and limbs exposed through wire-grid flooring. The results of an experiment which simulated indirect exposure of SPF mice to tolerantly infected wild mice supported earlier observations that without injury to the epidermis the risk of spread of infection from the infective urine, saliva or faeces is low.