BACKGROUND:A number of studies have used protein interaction data alone for protein function prediction. Here, we introduce a computational approach for annotation of enzymes, based on the observation that similar protein sequences are more likely to perform the same function if they share similar interacting partners. RESULTS:The method has been tested using interaction data about 3,890 protein sequences and averaging the results within protein families to account for over- and under-representation. For protein sequences that align with at least 40 sequence identity to a known enzyme, the specificity of our method in predicting the first three EC digits increased from 80% to 90% at 80 coverage when compared to PSI-BLAST. CONCLUSION:Our method can be applied not only to proteins for which we know interacting partners but also to their homologs. The method can be used for large-scale enzymatic functional annotation of protein sequences to refine predictions based on sequence matching alone, increasing the specificity of 10% of the predictions made by PSI-BLAST alone.