1. Most isolated guinea pig mononuclear exudative cells in tissue culture become typical migrating macrophages, but a small proportion take on fibroblastic characteristics, and produce pure colonies of fibroblasts. These fibroblasts maintain their morphological characteristics through repeated subcultures.

2. It is suggested that the subsequent development of individual mononuclear cells in tissue culture is conditioned at the time of explantation.

3. Apposition with other cells is not necessary for the initiation of mitotic cellular division.

4. There is a definite optimal relationship between the bulk of the medium, the number of explanted cells and the extent of proliferation. The presence of other cells in the vicinity enhances cellular division.

5. Mitosis in the isolated explanted cell is preceded by a latent period. The rate of division varies in different colonies of fibroblasts.

6. Admixed erythrocytes in the mononuclear suspension definitely inhibit proliferation of fibroblasts in tissue culture. The inhibiting factor in disintegrating erythrocytes is apparently present in the stroma.

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