(1) It is possible to detect lesions concentrating I131 with the scintillation counter described, provided that the concentration of activity in the lesion is 10 times that of normal brain, and the size of the lesion is greater than a certain minimum volume depending on its depth. If this depth is 5 cm. the limiting volume is 14 ml.; at 8 cm. depth (at the centre of the brain) it becomes 36 ml. (2) The counting rates observed can be related to the absolute uptake of activity in the normal brain and in the concentrating lesion expressed in μc/ml. (3) Where the counting rates are measured by means of a scaler, a counting period of 50 seconds at each standard position of measurement is adequate to reduce the statistical errors to a small fraction of the expected counting rates due to the concentrating lesions. However, the observed counting rates are sufficiently high to make possible the use of a ratemeter, and for speed and ease of measurement, this is recommended. (4) By improvements in counter design, the sensitivity of the method might be increased by a factor of two. (5) The syntheses of diiodo131 fluorescein and of dibromo82 fluorescein are described.