Digestion of adult glomerular basement membrane (GBM) with collagenase releases a number of peptides of which the noncoUagenous region of type IV collagen is detected by antibodies from patients with anti-GBM nephritis. In our study, 17 of 19 sera reacting with GBM, in indirect immunofluorescence, with cryostat sections of adult kidneys were negative with cryostat sections of fetal kidneys. However, after digestion with collagenase, a similar pattern of peptides was released from both fetal and adult GBM. SDS-PAGE immunoblotting revealed comparable antibody binding of all 19 sera with the peptides from either fetal or adult GBM. These findings suggest conformational differences between type IV collagen in fetal and adult GBM or masking of the antigen by other GBM constituents which may shield the antigen.