A decrease in the concentration of heparan sulphate proteoglycan (HSPG) in the glomerular basement membrane (GBM) is supposed to cause the increased GBM permeability in the congenital nephrotic syndrome (CNS). Therefore, we analysed the glycosaminoglycan (GAG) content and composition of the GBM and tubular basement membrane (TBM) from 3 patients with CNS of the Finnish type (FCNS) and 16 control infants. The GAG content, determined by spectrophotometric assay after papain digestion, was not significantly different in FCNS patients compared with controls. In addition, the GAG composition was comparable in the two groups, with heparan sulphate (HS) constituting at least 75% of the total GAG content. The urinary GAG content (expressed as mg GAG/mmol creatinine) was age dependent, but similar in both groups. Indirect immunofluorescence studies on kidney tissue from normal human infants, using monoclonal or polyclonal antibodies against the core protein of human GBM HSPG, showed linear staining of almost all renal basement membranes. A monoclonal antibody directed against the HS chain of HSPG showed strong GBM and a weak TBM staining. Kidney tissue from three patients with FCNS displayed no discernible differences in the distribution or quality of staining with the same antibodies. These biochemical and immunohistochemical results are in contrast to the decrease in anionic sites (by polyethyleneimine staining) and the replacement of GBM HS by chondroitin sulphate, observed by others in CNS of the diffuse mesangial sclerosis type.