Fibulin-1 and fibulin-2, two recently identified extracellular matrix proteins with a homologous domain structure, are known to bind various extracellular ligands and calcium. In this study, they have been localized at the light microscopical level in human embryos of gestational weeks 4-10, using polyclonal antibodies. Identical localization patterns were observed for the two fibulins in most of the tissues. In the heart, the endocardial cushion tissue and endocardium, but not the myocardium, were stained, as were the basement membrane zones and adventitia of blood vessels. Staining was also observed in the perichondrium and calcifying regions of developing bones. Moreover, reactions occurred with the gut subepithelium and epithelial basement membranes of the skin. Differences in staining patterns, however, were observed in various neural structures. Fibulin-1 was prominent in the matrix of the leptomeningeal anlage, in basement membranes of the neuroepithelium and the perineurium of peripheral nerves. Fibulin-2 was detected primarily within the neuropithelium, spinal ganglia and peripheral nerves. The early embryonic expression of both fibulins indicates specific roles during organ development and, in particular, involvements in the differentiation of heart, skeletal and neuronal structures.