E. A. Watson 
165 
Mix well and incubate for two hours at 37° C. 
The control set, tubes 10, 11, and 12, must not show any trace of 
haemolysis. 
The titre of the haemolytic serum is indicated by the amount present 
in the last tube of the series 1-9 in which dissolution of all the red cells 
is complete, that is, the least amount necessary to dissolve a definite 
amount of red cells. 
For example, if in tubes Nos. 1-6 haemolysis is complete, not quite 
complete in tube No. 7, and still less in Nos. 8 and 9, then the titre 
is the amount of serum in tube No. 6, or 1-0 c.c. of a 1: 2000 dilution, 
1 unit being expressed as 0-0005. 
A serum with a unit value of between 0-0002-0-0005 is quite satis¬ 
factory, but when the value of a single unit exceeds the latter amount 
the results are not so good. 
The relationship and combined action of haemolytic amboceptor 
and complement should be clearly understood. To do this a number 
of experimental tests should be undertaken, using in one series only 
one unit of amboceptor with fractional amounts of complement, in 
another series two units of amboceptor and lesser complement fractions, 
four units and so forth, progressively multiplying the number of ambo¬ 
ceptor units while further reducing the fractions of complement. It 
will be found, for instance, that two units of amboceptor require a 
lesser amount of complement than one unit to completely haemolyse 
a standard amount of red cells. The lesser the amount of complement 
that can be safely employed in the practical tests the more delicate 
becomes the fixation reaction, the equilibrium of the haemolytic system 
being more easily upset, even by a test serum naturally weak in antibody 
content and which, if a relatively large complement unit was employed, 
might be insufficient to give a complete reaction. On the other hand 
the reduction of complement must not be carried to such an extreme 
point that any slightly inhibitive property of one of the other reagents 
would tend to obscure it and give a false fixation. 
It is essential that for all subsequent titrations and tests a standard 
dose of haemolytic amboceptor be fixed and rigidly adhered to. For 
all practical purposes the use of two units of amboceptor permit of a 
sufficiently fine gradation of complement, while still allowing a margin 
of safety. The dose is therefore fixed constantly at two units, to which 
complement is always titrated as in the next procedure. 
Parasitology vni 
11 
