The relationship between mucosal mast cells (MMC) in the intestinal lamina propria, and globule leucocytes (GL) and granulated lymphocytes (GIEL) within the epithelium was compared by analysing the constituents of their granules. Glycosaminoglycan, as well as serine esterase, were present in the granules of all three cell types and the antigenic relationship of these serine esterases to MMC-specific protease (RMCPII) was examined. This enzyme was detected by immunocytochemistry within MMC and GL granules, but not in GIEL. The concentration of RMCPII in preparations of epithelial and lamina propria isolates correlated significantly with the numbers of MMC/GL present within these tissues. RMCPII was detected in epithelial isolates, and in preparations of isolated GIEL, only when MMC and GL were present. These studies add further support to the view that MMC and GL are of a common lineage and strongly suggest that GIEL are unrelated to MMC/GL.