Experiments are described which show that the rabbit papilloma virus elicits an antibody of one type only, this being capable both of neutralizing the virus and of fixing complement in mixture with it. The virus and its antibody have a powerful, specific affinity for one another, each being capable of absorbing the other in great excess when they are brought together in the test tube. The union formed by them in vitro cannot be dissociated in any demonstrable degree by dilution or centrifugation.

In many respects the findings differ from those with most viruses previously studied.

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