The action of human ascitic fluid and of dog serum on the immunization of fibroblasts in vitro was investigated. The toxic effect of ascitic fluid upon fibroblasts was slight, while that of dog serum was marked. When a small amount of antigen (1.5 to 2 per cent dog serum, or 8 per cent ascitic fluid) was present in the medium, the resistance of the fibroblasts slowly increased, reached its maximum after about 14 days, and gradually decreased. The curve expressing the phenomena closely resembles that of Jörgensen and Madsen showing the production of antibodies in an animal which received daily injections of antigen. When the antigen was present in the medium in a high concentration (from 5 to 8 per cent dog serum), the resistance reached its maximum in 4 days, and decreased rapidly.

It may be concluded that:

1. In the immunization of fibroblasts in vitro against a foreign protein, there is a relation between the amount of antigen, the time of the appearance of immunization, and its duration.

2. When a small amount of antigen is used, immunization slowly reaches its maximum, and slowly decreases.

3. When a large amount of antigen is used, immunization reaches its maximum in a short time, but the protection is of equally short duration.

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