It has been shown:

1. That complement exposed to ultra-violet light is not thereby sensitized to the action of heat (which indicates that it is not protein).

2. That inactivation of complement by ultra-violet light is accompanied by a decrease in its surface tension.

3. That photoinactivation of complement is not a result of any changes in hydrogen ion concentration since these are less than 0.05 pH.

4. That hydrogen ion concentrations high enough to transform serum proteins from the cation to the anion condition (i.e. past the isoelectric point) permanently inactivate complement.

These facts together with those given in previous papers lead to the following hypotheses.

1. That there is present in serum a hemolytic substance which is formed from a precursor (which may resemble lecithin) and is constantly being formed and simultaneously being broken down into inactive products.

2. That both precursor and lysin contain the same photosensitive molecular group.

3. That the lytic substance is dependent for its activity upon the state of the serum proteins.

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