A study was performed to correlate activity quantitation derived from external imaging with surgical tumor specimens in patients who received radiolabeled monoclonal antibody. Patients were given I-131 labeled 16.88 human antibody and scanned 3-5 times by planar and/or single photon emission computed tomography imaging methods to acquire time-dependent activity data in tumor and normal tissues. A method also was developed to assess the heterogeneous activity distributions in tumor samples. Postsurgical tumor and normal tissue samples were subdivided into volume elements (voxels) of 0.5 cm x 0.5 cm x 0.05 cm thick, which were used to verify the activity quantitation computed by the conjugate view method and to appraise the heterogeneity of radiolabeled antibody uptake. Through the use of the measured voxel activities, along with the time-dependent activity curves available for the entire tumor specimen derived from imaging, the cumulated activity and absorbed dose for each voxel were uniquely determined. The calculated total absorbed dose values were color-coded as isodose curves and overlaid on a correlated computed tomographic image. In two patients, activity quantitation derived from external imaging correlated with surgical tumor resection specimens within +/- 11%. The tumor-absorbed dose heterogeneity ratio was found to be as high as 10:1, with an average tumor to whole body absorbed dose ratio of 4:1. The mapping of activity with a histologic overlay showed a good correlation among activity uptake, the presence of tumor, and antigen expression on a microscopic scale. The resultant isodose curves overlaid on correlative computed tomographic scans represent the first images obtained with actual radiolabeled antibody biodistribution data in patients.