Quantification of the multifunctional cytokine Transforming Growth Factor-beta 1 (TGF-beta 1) in blood samples has aroused increasing interest in recent years, since an abnormal regulation of this cytokine appears to play a key role in the pathogenesis of different diseases; such as autoimmundiseases or malignant tumors. The measurement of TGF-beta 1 is complicated by a lot of problems concerning the collection, preparation and handling of blood samples, the platelet contamination, and the TGF-beta 1 activation procedure. Here, we recommend detailed instructions for measurement of TGF-beta 1 in blood plasma samples which should be followed to exclude the determination of false positive or negative results. (C) 1997 Elsevier Science B.V.