Glomeruli and tubules were isolated from rabbit kidney cortex by mild homogenization and sieving. Mixtures of these renal components were treated with detergents to prepare pellets of morphologically intact and easily distinguishable tubular (TBM) and glomerular basement membranes (GBM). These BMs were prepared for electron microscopy after: (1) no treatment; (2) treatment with buffer alone; or (3) treatment with enzyme (pronase, trypsin, pepsin, collagenase or testicular hyaluronidase). Mixtures of TBMs and GBMs were treated in the same incubation medium to keep constant the enzyme concentration, temperature, pH, and duration of the treatment. Untreated TBMs showed collapsed, highly folded sheets of electron-dense material. In contrast, control GBMs were thinner and strikingly resistant to changes in in vivo histoarchitecture. In all enzymatic treatments except hyaluronidase, TBM was more susceptible to digestion than GBM. In general, the effect of pepsin was greater than trypsin, which was greater than pronase. Collagenase also solubilized TBM but was only slightly effective in attacking GBM. Hyaluronidase-treated BMs were indistinguishable from controls. TBMs and GBMs were both affected least by the enzyme on their epithelial surfaces, which generally remained crisp and sharply demarcated. In contrast, fibrillar materials and BM fragments were released from connective tissue surfaces of TBMs and endothelial-mesangial surfaces of GBMs. Data in the present study indicate that various BMs are morphologically heterogeneous and that a “non-unitary” concept of BM is most appropriate. Moreover, the BM “sidedness” demonstrated following enzymatic digestions strongly suggests that macromolecular complexes within laminae densae may be arranged such a manner that opposing surfaces of the same BM are compositionally disparate.