Proteins of rat peritoneal mast cells (PMC), rat intestinal mast cells (IMMC) and human skin mast cells (SMC) were compared by two-dimensional electrophoresis. PMC and IMMC had many similarities in distribution of their neutral/acidic proteins but marked differences in the more abundant, granule-associated basic proteins. SMC proteins showed a unique distribution. Distributions of the products of in vitro translation of PMC and IMMC RNA were different from those of proteins isolated directly from these cells which, together with results of pulse-chase labelling experiments, shows evidence for processing of certain mast cell proteins including rat mast cell proteases I and II.