Quantitation from planar imaging and single photon emission computed tomography (SPECT) were compared in phantom studies and in patients receiving therapeutic doses of 131I-labelled anti-CEA. During the reconstruction of the data for SPECT quantitation attenuation correction and a correction for Compton scatter were used. The limitations of both methods were examined using the phantom studies and it was shown that practical SPECT quantitation could be achieved in patients given therapeutic doses of 131I-labelled anti-CEA, and furthermore, that SPECT appeared to give a more accurate estimate of the activity concentration.