1. Antisera were produced, separately, in rabbits to normal sap from healthy Turkish tobacco plants and to irus-sap from tobacco plants, affected with mosaic disease.

2. The immunologic reactions of the antisera were studied by means of:

(a) Alexin-fixation tests.

(b) Precipitation experiments, including: Precipitin-absorption tests with the same tobacco virus multiplied in tobacco, tomato, pepper, and petunia plants.

(c) Experiments with the inactivation properties of both antisera and normal serum on virus-sap.

3. From the results obtained from the above experiments, the following conclusions were drawn:

(a) Normal-tobacco-sap and virus-tobacco-sap possess antigenic substances in common.

(b) Normal sap and virus-sap of tomato, pepper, and petunia plants contain antigenic substances in common with normal sap of tobacco.

(c) Virus-saps of tomato, pepper, and petunia plants, have antigenic substances in common with virus-sap of tobacco, that are either not present in the normal tobacco sap or present only in small amounts.

(d) The two antisera possess alexin-fixing antibodies and precipitins in common.

(e) All of the precipitins to normal tobacco sap may be removed from either antiserum by absorption with virus-sap of tobacco.

(f) Specific precipitins for virus-sap of tobacco, tomato, pepper, and petunia are present in the antiserum to tobacco virus-sap and cannot be removed by complete absorption with normal sap of tobacco.

(g) Antiserum to virus-sap of tobacco, when used in an appropriate amount, has the power of completely inactivating virus-sap.

A corresponding quantity of antiserum to normal tobacco sap, or normal rabbit or guinea-pig serum, does not exhibit the same preventive action on virus-sap.

(h) There is some evidence that a specific antibody to virus-sap, lytic in nature, is present in the homologous antiserum.

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