The effect of peptides on the growth of ice crystals are studied using molecular dynamics simulations. The growth of the ice crystal is simulated at a supercooling of 14 K, and the effect of a single tetrapeptide on the growth rate is calculated. For pure ice the simulated crystal grows at a rate comparable to experiment. When a peptide molecule is added near the interface, the growth rate is diminished significantly, by up to a factor of 5 for Gly-Pro-Ala-Gly and a factor of 3 for Gly-Gly-Ala-Gly. The retardation occurs via the binding of the peptide to the ice surface, suppression of ice growth near the binding site, and eventual growth of the crystal around the bound peptide. The peptide with a proline residue is more effective in retarding the crystal growth, and this can be understood from the conformation of the peptide within the frozen ice phase after overgrowth. The simulations suggest that short peptides can be effective antifreeze agents.