The expression and distribution of S-100 protein and type IV collagen was studied immunohistochemically in sympathetic neuroblasts from the paravertebral region to the adrenal glands in human embryos and fetuses ranging from 7 to 12 weeks gestational age. From 7 weeks gestational age, S-100 protein was detected in round or oval cells mingling with sympathetic neuroblasts, and in spindle-shaped cells forming a continuous layer around them. The latter S-100 protein-positive cells were found in contact with the Schwann cells of nerve fibres entering the groups of sympathetic neuroblasts. Staining for type IV collagen showed that all groups of sympathetic neuroblasts were surrounded by a continuous basement membrane. By examining serial sections stained for type IV collagen and S-100 protein, a continuous basement membrane was found along the distribution pattern of the peripheral S-100 protein-positive spindle cells. The morphology of these cells, and their relationships with Schwann cells and with the basement membrane of the sympathetic neuroblasts, indicated that they were Schwann-like cells probably capable of synthesizing a continuous basement membrane separating the neuroblasts from the adjacent tissues. In contrast, the round or oval S-100 protein-positive cells, in contact with the sympathetic neuroblasts and not associated with nerve fibres, were considered as sustentacular or sustentacular precursor cells. At week 7 gestational age, the peri-adrenal sympathetic neuroblasts and their sustentacular and Schwann-like cells started to invade the adrenal glands and mingled with the adrenal cortical cells. These findings suggest the extra-adrenal origin of the sustentacular cells in embryonic and fetal adrenal glands.