Received 16. v. 62 l< xccpt for certain tumor viruses, the only experimental cvidencc published hithcrto which could indicate acquired immunological tolerance to a self-replicating agFnt consists of reports concerning thc virus of lymphocytic chorionicningitis (LCM). More than twenty years ago Traub (9) reported that if mice were infected with this virus intra utcro or shortly after birth they developed a lifelong, symptomless, virus carrier state. However, if mature mice were infected the infection would run a normal course, ending in either the death of the animal or recovery and clearance froin the organs-of any detectable virus. Further reports from Traub and others (lO,-ll, 12, 2, 4, 13) have described in more detail the LCM virus infections initiated in both mature and premature mice. The conclusion of all the reports seems to be that at least one important criterion for considering the non-responsive lifelong LCM virus carrier state to he due to tolerance is fulfilled, uiz. that the non-responsivc LCM virus infection can be brought about by premature exposure of the animals to the virus. However, a tolerance state is characterized not only by the fact that it can be brought about by premature exposure of the animal to an antigen, but that tolerance can also be terminated by adoptive inimunization, ie by injecting the tolerant animal with spleen and lymph node cells from normal, or even better, immune animals. Experiments concerning this second criterion for tolerance to the LCM virus have never been reported and therefore until more has been donc to clarify this problem doubt still would prevail as to the role played by the tolerance phenomenon in the LCM virus infection. Thc question whether an immunological tolerance to a virus can be dcvclopcd is of more than theoretical interest. Firstly, if a tolerance state to one virus really exists, this would point to the possibility of tolcrancc to other viruses. As it cannot bc excluded that in certain virus