The National Biomedical Research Foundation-Protein Identification Resource (NBRF-PIR) and the Protein Data Bank at Brookhaven National Laboratory (PDB) both contain protein sequences. We have prepared a cross-reference index of the sequences in these data banks, and compared the data. Of the 270 cases of sequences of the same protein appearing in both data bases, for only 31% are the sequences identical. This is often the result of a difference in the state of maturation of the proteins rather than experimental error. Nevertheless is useful to be aware that the sequence information in these two data archives should not be regarded as redundant.