AGf~ICULTURAL RESEARCH, PUSA.

214 PRACTICAL BAC1'ERIOLOGY
fixation of complement has occurred, so that not even
1 M.H.D. is left free. The serum and antigen controls
determine any possible anti-complementary action of
either of these reagents, and the complement dose
control indicates any possible deterioration in the
haemolytic value of the complement by dilution and
incubation. By carrying out the test in this quantitative
way the number of doses of complement
fixed by antigen plus serum can be estimatedi.e.
the strength of the reaction. Known negative
and positive sera must always be included in each set
of "Vassermann tests as controls.
The pract'ical details and interpretation of the results
can be lew'ned only by actual acquaintance with the
test.
It must be remembered that specimens of complement
from different guinea-pigs vary in fixability.
With a weakly fixable complement it may even be
impossible to elicit posit'ive reactions, while with a
strongly fixable complement non-specific effects arc
rendered morc marked. The controls with known
negative and positive sera are therefore indispensable.
Any anti-complementary effect of the serum or
antigen must be allowed for in estimating the
fixation by serum plus antigen, and the results with
known positive and negative sera are to be regarded
as standards for comparison with the results given by
the sera tested.
The criteria of positive and negative reactions
suggested by Browning and Kennaway may be
followed: a serum which permits of complete lysis
with the same minimum amount of complement as
the negative control serUlll is, of course, negative; a
SCl'11Ill whieh recpdres at least twice Hf> mnch complement
as the negative control serum to yield complete
lysis is to be regarded as positive. Sera which show
intermediate effects may be classified as "doubtful."
If the serum in question exhibits any degree of anticomplementary
effect, allowance must be made for
this.