Summary. The in vitro incorporation of 35S-methionine into hemocyanin was tested in the crayfish, Astacus leptodactylus. All organs assayed (hemocytes, heart, hypodermis, midgut gland, muscle and stomach) were able to synthesize proteins under our experimental conditions, but only midgut gland ( = hepatopancreas) incorporated labeled methionine into a protein which was preciPitated by an antibody against Astacus hemocyanin, and which comigrated with authentic hemocyanin on SDS gels.