The procedure used to determine the infective titer of the LDH agent, the reproducibility of this assay, and the relationship between virus dose and plasma enzyme activity were described. Multiplication of the LDH agent began within 6 hours after infection and reached 1010.8 ID50/ml of plasma within 24 hours. The titer rapidly decreased over the next 72 hours but viremia persisted for at least 16 months with titers as high as 105.2 ID50/ml. The appearance of the LDH agent in the circulation preceded the first noticeable rise in plasma LDH activity by close to 24 hours. After 10 months, when the plasma titer of the LDH agent had decreased nearly one millionfold, the plasma enzyme LDH had decreased by less than 50 per cent.
The LDH agent is inactivated by ether but withstands lyophilization, and freezing and thawing. It is stable at low temperatures. Ultracentrifugation at 105,000 G for 2 hours leaves less than 0.1 per cent of the LDH agent in the supernatant fluid and filtration through gradocol membranes suggesst that the upper size of the LDH agent is about 55 mµ.
Spread of the LDH agent from infected to uninfected mice kept in the same cage and transmission from mothers (infected prior to mating) to their offspring was relatively low.