Abstract: Rabbit peritoneal macrophages conditioned medium were collected and their effects on monocyte migration were investigated ,using micropore filter assay.The results showed that rabbit peritoneal macrophages could secrete a chemotactic factor for the monocyte.This factor was heat stable at 100℃ for 10 min,not dialyzable,but could be inactivated by trypsin and mercaptoethanol.These data suggest that this chemotactic factor is a peptide containing essential S S bonds.