1. When peritoneal exudates are produced in the rabbit by the injection of a large volume of isotonic saline, nearly all the cells in the exudate are polymorphonuclear leucocytes, and the number contained in a single exudate may exceed the entire number originally present in the circulation.
2. The migration of polymorphonuclear leucocytes from the blood stream into the exudate is followed by a stimulation of the marrow, so that the blood stream is filled with young cells, many of which also pass into the exudate. This marrow stimulation, with the resultant shift of the polynuclear count to the left, is probably produced by the absorption of breakdown products of the cells which first appear in the exudate.
3. If exudates are produced repeatedly at intervals of from 4 to 10 days, the marrow responds by throwing younger and younger cells into the circulation, so that the blood stream becomes full of very young polymorphonuclear leucocytes of class I, or even metamyelocytes. If 4 or 5 exudates are produced in rapid succession, the polymorphonuclear leucocytes in the circulation are all replaced by metamyelocytes and myelocytes.