Juvenile hormone (JH), ecdysterone and some antibiotics cause Tenebrio molitor to form a second pupa or pupal-adult intermediate. Incorporation of labelled leucine into the cuticular proteins of JH-induced second pupae did not differ from incorporation in normal pupae, and the soluble cuticular proteins from these young second pupae were indentical to those extracted from normal pupal exocuticle when analysed by SDS-polyacrylamide gel electrophoresis. However, as these second pupae aged, the major early bands did not undergo a normal decrease in staining intensity, indicating a JH effect on protein insolubilization (sclerotization). The transport of protein into the cuticle may also have been altered by JH; electrophoretic analysis of the new cuticle of treated animals showed intense staining of bands with RF's similar to those of blood proteins. The new exocuticle produced after treatment of pupae with ecdysterone had soluble proteins which were typical of normal pupae, but extracts from such animals aged prior to cuticle removal yielded bands characteristic of normal adults. Pupae treated with actinomycin D occasionally form new abdominal cuticle with characteristic pupal morphology. These cuticles yielded soluble proteins which, upon analysis, had pupal, pupal and adult, or adult banding patterns. Animals treated with mitomycin C, although retaining vestiges of pupal abdominal characters, had adult cuticular proteins.