Certain derivatives of benzacridine and of 4-anilinoquinoline give a blue or red colouration with a solution of iodine in aqueous potassium iodide. In the case of the active anilinoquinoline compounds the colour is usually developed in presence of solutions containing iodine at a concentration of the order of N/10,000, whilst in the case of the benzacridine compounds the colour is still apparent at concentrations of N/100,000 or even less. The effect of variation of concentration of compound, iodine, and hydrogen ions has been investigated in certain instances. The action of certain inorganic salts has also been investigated, but in low concentrations these are without much effect. The methosulphates of the two active benzacridine bases also develop colours with iodine even with very low concentrations of the latter. It is suggested that these methosulphates form micellar, colloidal solutions. The chromogenic property appears to depend not on the nitrogen atom in these compounds but on the structure of the molecule as a whole. The compounds which have been investigated differ from most of those previously known to give colours with iodine, in that they are basic and form colloidal solutions in which the particles are positively charged.