Mitosis was induced in macrophages from the peritoneal cavity of sensitized mice by subcutaneous injection of specific antigen. Animals were best sensitized by intramuscular injection of antigen emulsified in Freund's complete adjuvant, but were capable of giving a less intense mitotic response after sensitization by intraperitoneal injection of antigen in saline. Subcutaneous injection of fresh normal rabbit serum caused a small mitotic response, which was abolished by prior heating of the serum or absorption with mouse spleen cells. The changes in the peritoneal cell population during a mitotic response are described. Appearances suggested that mitosis occurred in both “old” and “young” macrophages, and that “young” macrophages were derived from lymphocytes by transformation. Mitosis was infrequent in these cells having morphology intermediate between lymphocytes and macrophages. Macrophage destruction apparently occurred only in the most intense reactions. Mitosis occurred when the peritoneal cavity was disturbed by repeated puncture of the abdominal wall. Mouse serum or plasma was found to be a potent stimulus to mitosis when injected intraperitoneally. Induction of a mitotic response by injection of soluble antigen in sensitized mice did not affect the growth of Listeria monocytogenes in the spleens of these mice or the phagocytosis of Salmonella typhimurium C5.