The utility of a gamma detecting probe (GDP) in the detection of experimental hepatic metastases in nude mice using radiolabeled monoclonal antibody (mAb) was assessed. Twelve mice with established hepatic metastases from the HT-29 LMM cell line, 5 mice with s.c. tumors in the left flank and 6 non-tumor-bearing control mice were given i.v. injections of 40 microCi/4 micrograms of 125I-labeled mAb HT-29-15. Six tumor-bearing mice were given i.v. injections of an isotype-matched control mAb (BL-3). Using the GDP, measurements were obtained daily over the region of the heart, the region of the liver (i.e., the right flank), and the s.c. tumor when applicable. Intraoperative measurements were obtained at laparotomy on days 5 and 7. Subsequently, the metastases, normal liver, and blood were resected and the radioactivity/g tissue was measured in a gamma well counter. External right flank/heart ratios were significantly higher in the tumor-bearing group than in controls. External measurements allowed detection of small tumors weighing only 161 +/- 87 (SD) mg and occupying 11.5 +/- 4% of the entire liver weight. Metastases counted intraoperatively with the GDP measured 1 to 7 mm in greatest diameter. The mean metastasis/heart GDP ratio was 1.7 +/- 0.4:1. Tumors weighing as little as 51 +/- 42 mg could be identified. These experimental results confirm the usefulness of the GDP for the detection of small hepatic metastases from colon cancer and illustrate important features of probe measurement of radiolabeled mAb uptake.