Three monoclonal antibodies were raised against a 200-kDa protein specifically expressed on the surface of hemocytes when Sacrophaga larvae pupated. One of these antibodies significantly inhibited dissociation of the fat body in the presence of pupal hemocytes, indicating that the 200-kDa protein is essential for dissociation of the fat body by the hemocytes. When the hemocytes were treated with a mixture of the monoclonal antibodies, they were found to secrete a significant amount of chymotrypsin-like proteinase. Moreover, the number of hemocytes expressing the 200-kDa protein increased significantly after puparium formation. These results suggested that some signal is transferred to the hemocytes via the 200-kDa protein when they interact with the basement membrane of the fat body and that this induces their secretion of a chymotrypsin-like proteinase essential for decomposition of the fat body.