Neutralizing titers of visna antisera measured in tissue culture were found to increase with the time and the temperature at which serum-virus mixtures were incubated prior to inoculation into culture tubes. The omission of preincubation resulted in neutralizing titers about 50 to 100 times lower than those observed after preincubation at 37°C for 3 hr.

The rate of neutralization of visna virus by antiserum was found to depend on the temperature and on the antiserum concentration, the rate being greatly reduced at increased serum dilution.

An exponential linear relationship between the amount of virus neutralized and the antiserum titer was found for virus doses up to about 104.5. The neutralization line had a slope of about 4.0, a 10-fold increase in virus dose corresponding to a 1.8-fold decrease in serum titer.

A virus fraction of about 10-5 to 10-4 could not be neutralized either by raising the antiserum concentration or by increasing the time of incubation at 22°C. The nonneutralizable fraction was found to be present in undiluted serum-virus mixture and it was not significantly increased by dilution, indicating practically no reactivation of neutralized visna virus.

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