Mast cell recovery in adult rats after depletion induced by polymyxin-B was studied. At days 17-18 after depletion the number of identifiable mast cells in the peritoneal fluid reached 3 .times. 106 cells/rat. Mast cell granule [MCG] constituents such as histamine, heparin, mast cell protease and .beta.-glucuronidase increased steadily during the observation period, whereas the amount of N-acetyl-.beta.-glucosaminidase decreased during the first 2 wk and was thereafter constant. At 2 wk after depletion the peritoneal mast cells were biochemically restored. From 14-34 days (end of experimental period) cell growth occurred with a simultaneous accumulation of the various MCG constituents.