1. The kinetics of processing newly synthesized bacterio-opsin from the non-crystalline state within the brown membrane to the crystalline state within the purple membrane was followed by pulse-chase experiments. 2. Biosynthesis of bacterio-opsin was found to be highly resistant to RNA-synthesis inhibitors like rifampicin and ethidium bromide. In the presence of ethidium bromide, only five protein species continue to be synthesized in halobacteria, one of them being bacterio-opsin. 3. In spheroplasts, synthesis of bacterio-opsin is found to be selectively disturbed. The purple membrane isolated from spheroplasts contains new, additional protein species with apparent molecular weights of 19 000, 23 000 and 29 000. These proteins share common amino acid sequences with bacterio-opsin. 4. In the halobacterial cell membrane, two membrane proteins with apparent molecular weights of 30 000 and 36 000 were detected which are structurally related to bacterio-opsin. 5. Bacterio-opsin as well as the 30 000 Mr and 36 000-Mr proteins contain covalently bound sulphate.