6-[(4,5-Dihydro-1H-imidazol-2-yl)amino-]-7-methyl-1H-benzimidazole-4-carbo- nitrile substantially free of 2,3,7-triamino-4,6-dimethyl-1,9-phenazinedicarbonitrile, and the anhydrous monoacetate salt thereof, are useful in the treatment of alpha-2 mediated disorders such as ocular hypertension.