It may be concluded that, under the conditions of the experiments:

1. A small amount of foreign protein added to the culture medium does not modify the rate of proliferation of fibroblasts.

2. A large amount of foreign protein added to the culture medium decreases markedly the rate of proliferation of fibroblasts cultivated previously in homogenic medium, while it does not decrease the rate of proliferation of fibroblasts cultivated previously in the presence of a small amount of the foreign protein.

3. Fibroblasts in vitro respond to the presence of an antigen in the culture medium by becoming immunized against its action.

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