Frozen sections of human fetal kidneys were studied for the appearance of vascular elements to the developing glomeruli using Ulex europaeus (UEA-I) lectin as a marker for endothelial cells. Chains of UEA-I-positive cells, seen to extend from larger vessels, could be observed among the uninduced cells of the nephrogenic mesenchyme. During the S-shaped body stage of nephrogenesis, the cleft of the comma-shaped cluster of epithelial cells was invaded by the UEA-I-positive cells, and during further glomerular development, the UEA-I-positive cells were seen to grow in number, prior to the appearance of visible lumina to the capillaries. The present results show, in addition to revealing the vascularization pattern of developing glomeruli, that endothelial cells obtain some of their characteristic glycoconjugates during early stages of differentiation. Thus, UEA-I seems to be a valuable tool to study in detail the vascularization of various developing human tissues.