On each of 2 occasions when a tolerant lymphocytic choriomeningitis (LCM) virus infection was introduced experimentally by a natural route into breeding stock of the Pirbright P(SD) mouse strain, and a comparison made with genetically identical controls, where was in successive generations an overall 15-fold increase in the incidence of lymphomas. Most of these were recognized clinically after the age of 16 months. The same observation was made in mice of the P(H) strain from which the P(SD) strain had been surgically derived. The finding is interpreted as the activation of an endogenous viral oncogene by the LCM infection. It was concurrently observed that the incidence of mammary tumours in the P(H) strain mice before the age of 16 months was markedly lower in the animals infected with LCM.