1. Rabbits which have received a single dose of normal horse serum in the subarachnoid space produce precipitins in the blood in greater abundance, of higher titer, and persisting longer than those in control rabbits which have received a similar injection intravenously.
2. Repeated subarachnoid injections of normal horse serum in rabbits induce precipitins in the blood early. These may appear in high titer as soon as 1 week after the initial injection, whereas in rabbits similarly treated intravenously no precipitins are found at this time. They may appear a few days afterward and reach a high titer.
3. No anaphylactic manifestations occurred in rabbits treated repeatedly with subarachnoid injections of normal horse serum when the precipitin content of the blood was high.
4. Anaphylactins, as determined by passive transfer of anaphylaxis, were demonstrated in sera with high precipitin content.
5. These experiments may explain clinical evidences of anaphylaxis, observed when an initial intravenous injection of horse serum followed a series of intraspinal injections of such serum.