The [ 3H]phenylalanine-containing peptides synthesized from l-[ 3H]phenylalanyl RNA in a cell-free system from Escherchia coli, containing poly (uridylic acid), elongation factors and GTP at 10 m m Mg 2+, have been separated after alkaline treatment into three fractions: phenylalanylphenylalanine (Phe) 2, oligophenylalanine [Oligo-(Phe)] and polyphenylalanine [poly(Phe)]. Under conditions at which in the absence of inhibitors peptide chain elongation proceeds so that (Phe) 2 and oligo(Phe) constitute a substantial proportion of the total Phe-containing peptides formed, amicetin, lincomycin, spiramycin and erythromycin affect the formation of (Phe) 2 to a much smaller extent than they do the formation of oligo (Phe) and poly(Phe). On the other hand, thiostrepton and siomycin A cause a decrease in (Phe) 2 while affecting poly(Phe) formation to a much lesser extent; this result parallels that obtained with tetracycline in the same system [ Biochim. Biophys. Acta 240, 137; 247, 632 (1971)], and is compatible with the proposal that thiostrepton and siomycin A inhibit primarily the enzymic binding of aminoacyl RNA to the ribosome.