Histamine dihydrochloride (HC) is one of the bronchoconstrictors used for bronchial challenge. Information on the activity of HC dilutions during storage is desirable (4). Activity, bacterial, and fungal contamination of stored HC dilutions were tested after storage at 20 degrees, 4 degrees, and -18 degrees C. HC dilutions with a concentration of and below 0.25 mg/ml have a significantly reduced activity after 1 month's storage at 20 degrees C and should at present be used within 1 week to ensure the presence of the expected activity. The activity of HC dilutions stored at 4 degrees C or -18 degrees C was stable for at least 6 months. Upon delivery from a pharmacy we detected no bacterial or fungal contamination of HC dilutions by means of the method used. Before 3 months' storage, and independently of storage temperature, bacterial contamination was not found. After 3 months' storage, bacterial contamination was found in HC dilutions with a concentration below 0.5 mg/ml. No fungi were isolated. HC dilutions with a concentration of 0.25 mg/ml and below should not be stored for more than 1 month and should be used within 1 week of opening.