Poly(2'-fluoro-2'-deoxyadenylic acid), poly(2'-chloro-2'-deoxyadenylic acid) and poly(2'-bromo-2'-deoxyadenylic acid) are used as messenger RNAs in protein synthesizing systems in vitro. All polynucleotides were active as messengers and [14 C]lysine was incorporated into polypeptides. The initial velocity of polylysine formation was greater using poly(2'-fluoro-2'-deoxyadenylic acid) as messenger than in the case of poly(rA), and all synthetic messengers lived longer in the protein synthetic system.