The reactivity of 10 human anti-glomerular basement membrane (GBM) autoantibodies with basement membrane antigens of human adult and infant kidney, lung, placenta, and skin was examined by ELISA and immunofluorescence microscopy. All autoantibodies were previously shown to react with adult kidney by indirect immunofluorescence and with collagenase-digested adult GBM by ELISA. Four antibodies (group A) were positive on infant and fetal kidney sections by immunofluorescence, and six antibodies were negative (group B). By ELISA both groups of antibodies were reactive with collagenase digests of infant GBM. After denaturation with 6M urea (pH 3.5) infant and fetal kidney sections reacted with group B autoantibodies by immunofluorescence, which indicated that the antigen(s) was masked. Hidden antigenic determinants in lung, placenta, and skin were reactive with groups A and B autoantibodies only after acid urea denaturation of tissue sections. Within each group variability in reactivity of autoantibodies with basement membranes suggested further heterogeneity.