The effect of BF3-methanol treatment on the mass and fine structure of isolated Clivia leaf cuticles at different stages of development has been investigated. BF3-methanol cleaves ester linkages in cutin; however, the cuticles are not completely depolymerized. With increasing age, the residue left after BF3-methanol treatment increases in mass. In very young cuticles, 10% of the total cutin resisted BF3-methanol and the fraction of nonester cutin increased up to 62% in mature leaves. Transmission electron microscopy shows that fine structure of the cuticle proper is severely distorted but not destroyed. The internal cuticular layer, which exhibits a heavy contrast when fixed with KMnO4, is completely depolymerized, while the external cuticular layer is hardly affected. The results are discussed in relation to cuticle development and to the function of cuticles as transpiration resistances.