A strain of lymphocytic choriomeningitis virus has been encountered, which grows readily in mouse embryo, serum, Tyrode culture media. Its origin is not definitely known but appears to be either the mouse brain tissue or, more probably, the monkey serum.
This strain gives clear cut results on filtration tests through Elford membranes, establishing the size of the virus, according to formula, as 33 to 50 mµ.
The strain shows a high and uniform virulence in W-Swiss mice. This appears to be due in part, at least, to the age and strain of mice employed for passage and titration.
The strain has been found to be more virulent in young than in old mice, especially following intraperitoneal inoculation.
Finally, the strain, when given as a vaccine intraperitoneally in amounts as small as 160 intracerebral lethal doses, induces an immunity against subsequent intracerebral inoculations of as much as 10,000 lethal doses.