The development of targeted therapy requires that the concentration of the therapeutic agent can be estimated in target and normal tissues. Single photon emission tomography (SPET), with and without scatter correction, and planar imaging using 131I have been compared to develop a method for investigation of targeted therapy. Compton scatter was investigated using line spread functions in air and water, these data were used to set a second peak, adjacent to the photopeak, for scatter correction. The system was calibrated with an eliptical phantom containing sources in background activity of various intensities. Scatter corrected reconstructions gave accurate estimates of activity in the sources regardless of background activity. For planar scanning and SPET without scatter correction there was an overestimate of activity in the source of 290% and 40% respectively. The validity of this method was confirmed in patients by comparing activity in the cardiac ventricles measured by SPET with scatter correction with that in a simultaneous blood sample. A coefficient of correlation of 0.955 was achieved with 25 data points. SPET with scatter correction was compared with planar imaging in measuring activity in the liver and spleen of patients receiving 75 mCi 131I-antibody to CEA intravenously. Planar imaging gave significantly higher values than SPET for the spleen (t = 5.4, P less than 0.001 by the paired t-test) but no significant difference for the liver. SPET with scatter correction forms a basis for an improved technique of quantifying the targeting efficiency.