As a part of the study on the relation between exogenous polyamines and various components necessary for protein biosynthesis in the germinating maize seeds, the effects of the polyamines on protein biosynthesis and irbosome activity were investigated. The protein biosynthesis activity by S-30 containing all components necessary for protein biosynthesis was increased by exogenous polyamines, spermidine, spermine and putrescine. As the concentration of polyamine treated was increased, the optimal Mg2+ concentration of in vitro poly U-dependent protein synthesis system was gradually reduced. However, the optimal Mg2+ concentration of poly U-dependent system containing optimal polyamine was 10 mM regardless of the sort of polyamine. It could be infered that polyamines play an important part in protein biosynthesis in the higher plant, and that the role of polyamines take partially the place of Mg2+ action. The activities of ribosome and S-100 in protein biosynthesis were increased by 46.7% and 17.7% with spermidine, and by 44.1% and 16.2% with spermine, and by 29.1% and 19.3% with putrescine. It could be concluded that the increase of protein biosynthesis by polyamines in mainly owing to the ribosome activation.