20 subjects (8 men, 12 women) with anaphylactic or urticarial reactions after drug intake were evaluated. Tryptase (TRY) and eosinophil cationic protein (ECP) were determined in serum 2 and 24 h after the allergic episode. Histamine was determined in three sequential urine samples. Results indicated that in the group of subjects evaluated, TRY and histamine were elevated, although in not all cases for both markers. With ECP, no clear association was found either at 2 or 24 h after the allergic episode. These results suggested that mast cells participated in the adverse reactions, but a role for eosinophils could not be established. Further studies need to be undertaken to establish the participation of these cells in immediate reactions to drugs.