THE ROCKEFELLER INSTITUTE
FOR MEDICAL RESEARCH

DEPARTMENT OP ANIMAL AND P- PATHOLOGY
       PRINCRTON, NEW JERSEY

May 7, 1936.

Dr. Michael Heidelberger,
,The Bo&efeller Institute for Medical Research,
66th Street and Iork Avenue,
low York, I. Y.

Dear Dr. Heidelberger :

  In my study of the neutralization of plant viruses by imsuue serum,
I have regularly fourd that, although It is comparatively easy to neutralire
met of the virw In a given sample of Juice, it is very difficult or in+
possible to neutraliss so completely that no unueutralised virus whatever
can be demonstrated (our methods for detecting small quantities of virw
are highly sensitive). In this respect, the neutralisation of plant viruses
by serum appears to be governed by a mass actfon relationship such as one
sees in the neutralisation of tetanolysin by its antitoxin.

  This situation in plant virus neutralization appears very aualogous
to your findings with the precipitln reaction in which the most careful
titrations show presence of small amounts of antibody and antigen in neutral
mixtures, as in the dissociation of certain salts. A question which has
arisen concerns the problems of whether a small amount of dissociation regue
larly occurs In what are customarily considered neutral mixtures of antigen
and ant ibody.  Since you have been interested in this phase of serologJT, I
wonder if you could direct me to references in the literature which handle
this problem critically.

  You may recall that when you visited Princeton I told you of a method
which has recently been worked out for recoverfng large quantities of virus .
from neutral antibody-virus precipitates.  At that time you regretted that
the methods did not permit the reciprocal type of experiment, recovery of
antibodies from such precipitates.  Recent experiments have indicated that
this latter Is possible with certain of the plant viruses.  One of the potato
viruses, for exsmple, is highly susceptible to acid, while its antibody is
relatively resistant.  If washed precipitate'from this virus aud its antibody
is acidified, the virus undergoes dissolution and virtually lOO$ of the antl-
bodies are recoverable in relatively pure state.

Very sincerely yours,

K. Starr Chester,

Ksc :YS