I. Introduction Of the ever increasing number of viruses known to affect man and higher animals, the virus of lymphocytic choriomeningitis (LCM) was one of the first to be discovered. Indeed, this virus has been known and maintained in the laboratory by passages in a relatively simple host, the mouse, for 35 years. Yet our knowl- edge of its properties is still scanty when compared with the wealth of informa- tion available for other viruses, some of which have come to our attention much more recently. There are at least four reasons which may help to explain this seeming paradox. (1) The...