Protein modifications such as phosphorylation are often studied by two-dimensional gel electrophoresis, since the perturbation in the protein's pI value is readily detected by this method. It is important to be able to calculate the changes in the pI values that specific post-translational modifications cause and to visualize how these changes will effect protein migration on 2D gels. To address this need, we have developed ProMoST. ProMoST is a freely accessible Web-based application that calculates and displays the mass and pI values for either proteins in the NCBI database identified by accession number or from submitted FASTA format sequence.