The photoinactivation of complement has been studied with a view to determining if possible how many kinds of molecules disappeared during the reaction. It was found that:

1. The apparent course of photoinactivation is that of a monomolecular reaction.

2. Diffusion is not the limiting factor responsible for this fact, because the temperature coefficient of diffusion is much higher than that of photoinactivation (Q10 = 1.22 to 1.28, and Q10 = 1.10 respectively).

3. There is no change in the transparency of serum solutions during photoinactivation, at least for light of the effective wave-length, which is in the ultra-violet region probably at about 2530 Ångström units.

It is pointed out that under these conditions only one interpretation is possible; namely, that during photoinactivation a single disappearing molecular species governs the rate of reaction. This substance must be primarily responsible for the hemolytic power of serum when it is used as complement.

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