A new method has been developed for the coupling of diethylenetriaminepentaacetic acid (DTPA) to proteins using the cyclic anhydride of DTPA. The anhydride, prepared by a simple one-step synthesis, is added as the solid to the solid protein. Coupling is completed in minutes at room temperature following the addition of aqueous pH 7 buffer. The coupling has been characterized by the use of exhaustive dialysis, gel chromatography, and u.v. spectrophotometry. Under optimal conditions of anhydride: protein molar ratio and protein concentration, up to 70% of the DTPA groups may be coupled to protein. Essentially all free DTPA may be removed from DTPA-coupled albumin preparations by a single passage through a 20-cm gel filtration column. Biodistributions in normal mice at 45 min obtained for 111In-albumin showed 33 +/- 1% injected dose per gram in blood compared to 35 +/- 3% for 125I-albumin. Results for all tissue samples are in agreement within two standard deviations. The simplicity with which albumin has been coupled with DTPA by this method contrasts sharply with existing methods and is an attractive area of research for the labeling of a variety of proteins with a variety of metallic radionuclides.