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 early belief that the LCM virus was the frequent cause of human diseases had soon to be abandoned; infections of man with this virus are rare. (2) By way of contrast, laboratory infections are not uncommon and they frequently run severe and even fatal courses. (3) Until recently, the only means of titrating the virus was by mouse inoculation, a method in which accuracy and economy are poorly correlated. (4) The virus is of unusual lability, being quickly inactivated under conditions which leave other viruses intact. Thus, when balancing medical and theoretical importance against personal hazard and tech nical difficulties, the result was quite unfavorable, and lack of interest was really not surprising. In the last few years, however, the situation has gradually changed and an increasing number of workers have turned their attention to this virus.