Abstract: It has been reported that harringtonine inhibits protein synthesis in ceil-free system. In order to study the mechanism of action of harringtonine on protein synthesis, effect of the drug on the initiation and completion of globin synthesis in rabbit reticulocyte lysate and formation of peptidyl-³H-puromycin on rat liver polyribosome were investigated.The results showed that: (1) there was a delay of several minutes in the onset of inhibition following addition of harringtonine to the lysates; (2) complete ³H-globin chains synthesized after the addition of harringtonine were identical to those synthesized in the absence of the drug representing peptides that have passed the initial stages of synthesis when harringtonine was added; (3) formation of peptidylpuromycin complex was not inhibited by harringtonine. These results suggest that harringtonine inhibits protein synthesis at one or more steps prior to the formation of the first peptide bond.