Fourteen natural products, known to inhibit other proteins of the Zincin-like fold class, were screened for inhibition of the Zincin-like fold metalloprotease thermolysin using mass spectrometry. Fourier Transform Mass Spectrometry was successful in identifying actinonin, a known inhibitor of astacin and stromelysin, to be an inhibitor of thermolysin. Molecular modelling studies have shown that specificity within the Zincin-like fold is determined by Protein Fold Topology.