A short form of Escherichia coli translational initiation factor IF3, repeatedly found both in vivo and in vitro, lacking the positively charged N-terminal hexapeptide has been produced by mild trypsinization. The properties of this short form of IF3 have been studied. Compared to the long native form of the factor, the shortened IF3 displays a markedly decreased thermal stability and affinity for the 30 S ribosomal subunit, as well as a reduced biological activity in protein synthesis. Following the loss of the N-terminal hexapeptide, a second peptide bond (Lys-90-Val-91) becomes easily accessible to proteolytic attack suggesting that formation of the short IF3 may be the first step in the physiological degradation of the factor.