The cuticles from distinct developmental stages of Ascaris suum were isolated by a combination of mechanical disruption and detergent treatment of larvae or by surgical removal of cuticle from adults. Proteins from the isolated cuticles were solubilized with SDS and 2-mercaptoethanol (2ME) and analyzed by PAGE. Cuticular proteins from the third and fourth larval stages (L3 and L4) were comparable to adult, but differences in the number of bands were observed. The soluble proteins from the adult, L3 and L4 were readily degraded by bacterial collagenase, suggesting that these proteins are collagen-like structural elements of the cuticle. The soluble proteins from the L2 differed from the adult and other larval stages in both the number and molecular weight of protein bands and their lack of collagenase sensitivity. Antibodies made against the soluble cuticular proteins reacted with the medial and basal layers of the cuticle but not the external cortical or epicuticular regions. A significant amount of the cuticle was not solubilized by 2ME and was not digested by bacterial collagenase. These insoluble cuticular proteins were probably derived from the epicuticular and external cortical regions of the cuticle. Different developmental stages of A. suum were biotinylated and examined by electron microscopy. An organic soluble biotin reagent labeled all stages in a transcuticular pattern, while an aqueous soluble biotin labeled only the external cortical and epicuticular regions of the L4 and adult cuticle. These data indicate the presence of a hydrophobic barrier in the cuticle of later stages of the parasite.