1. Guinea pigs injected intracutaneously and subcutaneously with extract of the pollen of burweed marsh-elder in relatively small amounts did not show anaphylactic response to intravenous shock doses of this material 3 weeks later.

2. If, however, the animals were sensitized with horse serum either before, or along with the same pollen injections, they could then be shocked after an interval of 3 weeks with pollen extract alone.

3. The possible rôle of this underlying sensitivity is discussed.

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