1. A procedure is described for the detection and assessment of informational complementarity in an amino acid sequence; it is based on possible autocomplementarity in the mRNA, and involves codon-to-codon matching. 2. This procedure was applied to myelin basic protein, a variety of protamines, histone IV, silk fibroin, rat skin collagen alpha1 chain and a sheep keratin. A multiplicity of extensive low-probability informational symmetries, based on codon-to-codon matching, were detected. 3. These low-probability orderings, which are independent of the actual mRNA codons, are rationalized in terms of the evolutionary ordering of the amino acid sequences concerned, in such a way that constraints on the secondary structure of the coding polynucleotides were satisfied. This possible interpretation is supported by a number of significant common properties of the protein sequences analysed.