Using indirect immunofluorescence, cryostat sections from renal biopsy specimens of 14 adult patients showing marked diffuse thinning of the glomerular basement membrane (GBM) on ultrastructural analysis were examined for the presence of the Goodpasture (GP) epitope M2 using anti-M2 antiserum. In no case was a total absence of M2 noted. The fluorescence pattern was fine but homogeneously linear along the GBM in 12 cases, intensity varying from +-++, as compared with for the control specimen GBM. A faint, broken line of stain, intensity+, was observed in biopsy specimens of two patients, one of whom had family members with progressive hereditary nephritis, type Alport's syndrome. Clinical presentation was dominated by hematuria (10/14 patients) but also included three patients with isolated proteinuria. Two patients had nephrotic range proteinuria. Other than the GBM changes, histological findings were sparse, with either no abnormalities or only slight mesangial increase in most. One case of focal segmental sclerosis and hyalinosis was also found. The findings from this study suggest that the abnormally thin GBM does not lack the GP epitope, but it may be reduced.