Summary Three series of experiments are described showing the effects of Dimidium Bromide (1553) against T. congolense infection of Zebu cattle. In the first and second series the high trypanocidal effect of the drug is demonstrated. In the third series, results are described which suggest that the use of the drug may be contributory to certain unpleasant delayed sequelae which have been likened to the syndrome of photosensitisation. The experiments are discussed and it is concluded that the drug is an effective trypanocide, but that its use is attended by the risk of delayed photosensitisation. This is attributed to a build-up of photodynamic substances in the blood after liver dysfunction has been caused by Dimidium Bromide. A suitable dose for field use is suggested and a warning regarding the instability of solutions of the drug is given.