We reported elsewhere (Varenne et al., 1982) that, during synthesis of a number of colicins in Escherichia coli, intermediate nascent chains of discrete sizes accumulated, suggesting a variable rate of translation. In this paper, a detailed analysis provides arguments that this phenomenon, at least for the proteins under study, is not related to aspects of messenger RNA such as secondary structure. It is linked to the difference in transfer RNA availability for the various codons. Experimental analysis of translation of other proteins in E. coli confirms that the main origin for the discontinuous translation in the polypeptide elongation cycle is the following. For a given codon, the stochastic search of the cognate ternary complex (aminoacyl-tRNA-EF-Tu-GTP) is the rate-limiting step in the elongation cycle: transpeptidation and translocation steps are much faster. The degree of slackening in ribosome movement is almost proportional to the inverse of tRNA concentrations. The verification of this model and its possible physiological significance are discussed.