Rat peritoneal mast cells were separated by using density gradients of PVP[polyvinylpyrrolidone]-coated silica particles (Percoll). Mast cells were isolated on preformed Percoll gradients or cell separation was made simultaneously with the gradient formation. Both procedures resulted in mast cell suspensions of 95-99% purity. As tested by Ruthenium red staining and EM, the isolated mast cells showed a very good preservation of cell structure and reacted easily to the degranulating agent Compound 48/80. Practically all mast cells could be recovered from the peritoneal cell suspension. Percoll was superior to earlier isolation procedures by giving a practically pure and intact mast cell suspension and by avoiding cell aggregation.