[en] To label proteins with positron emitters with a half-life in the order of days, a method has been developed to label proteins with zirconium (Zr)-isotopes. Therefore, the bifunctional chelating agent desferal (Df) was coupled to albumins via a thioether bond. Labeling of the premodified proteins was easily performed by addition of these proteins to freeze-dried Zr-oxalate. This labeling was efficient (>90%) and accomplished in several minutes. The conjugates showed a high in vitro stability. Biodistribution studies were performed with ⁸⁸Zr-citrate, ⁸⁸Zr-Df, and ⁸⁸Zr-labeled mouse serum albumin (⁸⁸Zr-Df-MSA), modified with different amounts of chelating groups. Whereas Zr-citrate was found to accumulate in bone, Zr-Df was cleared very fast by glomerular filtration. The ⁸⁸Zr-Df-MSA showed similar blood clearance as did ¹²³I-labeled MSA. The biodistribution pattern of ⁸⁸Zr-Df-MSA differed only from ¹²³I-MSA in that a higher accumulation of Zr in liver, kidney, and spleen was found. The absence of large amounts of ⁸⁸Zr in bone indicated that in vivo the conjugates are also reasonably stable