One hundred six immunoincompetent mice grafted with human parathyroid adenoma or carcinoma were used to evaluate distribution of the murine monoclonal antibody E11, which recognizes a calcium sensor of high molecular weight on the parathyroid cell surface. The subcutaneous parathyroid grafts were infiltrated with murine fibrous tissue, which seemed to increase with the duration of transplantation and the size of inserted tissue pieces. Intraperitoneal injection of biotinylated or 125I-labeled E11 antibody indicated time- and dose-dependent antibody accumulation, as well as the presence of unoccupied binding sites in the transplanted parathyroid tissue. The iodinated intact immunoglobulin G and Fab fragment of the E11 antibody demonstrated low radioactivity in the lung, liver, spleen, kidney, and intestine for up to 14 days, except for the Fab fragment, which was rapidly accumulated and cleared from the kidney. The peak radioactivity ratio in the adenoma tissue versus blood averaged 2.8 for the intact antibody and 5.3 for the Fab fragment, whereas the corresponding values for the carcinoma tissue were 8.6 and 8.8, respectively. These ratios increased considerably, especially for the adenoma specimens, when weights of the excised grafts were adjusted for the calculated content of parathyroid tissue. The results support that the E11 antibody may localize even minute amounts of human parathyroid adenoma and carcinoma tissue.