Human recombinant tissue plasminogen activator (tPA), labeled with 131I(1.1 to 6.2 mCi) by the iodogen method, was administered intravenously to 15 patients with various soft-tissue malignant tumors after blocking of thyroidal radioiodine uptake. Gamma camera imaging was performed 4 and 24 hr after injection; three patients were also imaged 5 days following injection. We observed accumulation of radioactivity in primary and secondary lesions in 11 patients. In this preliminary study we did not detect any definite association between the magnitude of uptake and type of tumor. Tumors were usually visualized already after 4 hr but the uptake was more intense at 24 hr. The target-to-nontarget ratios at 24 hr, determined by computer analysis of stored images, varied from less than 1.2 to 2.1. This is the first demonstration of accumulation of radiolabeled tPA in malignant tissue. We do not know the mechanism of the uptake but because tPA is known to be avidly bound to fibrin, a component of the stroma of many malignant tumors, it is possible that [131I]tPA is bound to fibrin rather than taken up by the malignant cell; various possible cell uptake mechanisms are discussed. Due to the relatively early maximal uptake of this radiopharmaceutical it will be possible to substitute 123I for 131I, a possibility suggesting a potential clinical use of radioiodinated tPA for the detection of malignant tumors of various origin.