Spinach leaf (Spinacia oleracea L. var. Kyoho) protoplasts sustain protein-synthesizing activity as measured by the incorporation of [(14)C]-leucine into the protein fraction both in the light and in the dark. By the immunoprecipitation of ribulose-1,5-bisphosphate (RuP(2)) carboxylase with rabbit antibody raised against the purified spinach enzyme preparation, it was found that approximately 7% of the total radiocarbon incorporated into the protein fraction in the light was in the carboxylase molecules. However, there was no measurable net increase observed in the content of the enzyme protein in the experimental conditions employed. It was found that both chloramphenicol and cycloheximide inhibited the incorporation of [(14)C]leucine into RuP(2) carboxylase and its constituent subunits, as measured by the immunoprecipitation of the enzyme molecule and its subunits, A and B.