The Wilms' tumour (WT) is composed of blastema, epithelium and mesenchyme; the epithelium and possibly also the mesenchyme develop from the blastema, parallel to embryonal development. Since interactions between cell adhesion receptors and extracellular matrix (ECM) proteins play an important role in tissue maturation, we examined the expression of the integrin subunits alpha 1-alpha 6, beta 1 and beta 4, and of the ECM proteins fibronectin, laminin and collagen I and IV, in 20 frozen WT samples and in 5 fetal and 2 adult kidneys. The integrin and ECM protein distribution in tumour epithelium and mesenchyme showed strong similarities to that in their fetal counterparts, whereas the tumour blastema differed strongly from the fetal blastema. In the WT blastema different components were recognized Undifferentiated blastema, characterized by expression of alpha 3 and alpha 6 and the virtual absence of ECM proteins. Blastema with epithelial commitment, showing increased expression of alpha 3 and alpha 6 and the appearance of alpha 2 and, as a very early phenomenon, production of laminin. Blastema with mesenchymal commitment, with loss of alpha 3 and alpha 6 and expression of alpha 1, alpha 4 and alpha 5 and presence of ECM proteins. It is speculated that the inability of the (undifferentiated) blastema to produce ECM proteins is related to its relatively high metastatic potential when compared with epithelium and mesenchyme.