Total RNA was isolated from cyanoblast-containing tissue taken from behind the compound eye of the horseshoe crab, Limulus polyphemus. Poly(A)-containing RNA separated from this by affinity chromatography on oligo(dT)-cellulose was translated in the rabbit reticulocyte haemolysate system in the presence of L-[35S]methionine. By using an antiserum to Limulus haemocyanin, polypeptides were isolated from the translation products which had a similar mobility to the authentic Limulus haemocyanin polypeptides as judged by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis.