Histidinol dehydrogenase from three differing revertants of ICR-191A-induced frameshift hisD3018 has been purified and examined for amino acid replacements. The enzyme from one spontaneously arising revertant, R7, contains an extra proline residue, whereas that from another, R5, contains an extensive frameshifted sequence, four amino acid residues of which have been identified to date. The amino acid replacement data are in agreement with the in vitro code word assignments and allow the characterization of the hisD3018 frameshift as an addition of one nucleotide pair, most likely guanine plus cytosine. Enzymatic data for those ICR-191A-induced revertants of hisD3018 arising within the hisD gene indicate that the enzyme is wild type and, therefore, that ICR-191A can cause deletions as well as additions of single base pairs. The wild-type amino acid sequence is restored in enzyme from an N-methyl-N'-nitro-N-nitrosoguanidine (NG)-induced revertant, R29, suggesting that NG is a base-deleting as well as a base-substituting mutagen. The unusual response of hisD3018 to external suppressors is considered in terms of reinitiation of protein synthesis out of phase, coupled with suppression of a nonpermissive missense codon so generated, and of an alternative hypothesis invoking a true frameshift suppressor transfer ribonucleic acid with an extended or deleted anticodon.