A number of α-amino acids, simple peptides, and some sulfur-containing compounds have been assayed as inhibitors of calcification of costal cow cartilage. In general, the α-amino acids increased calcification, whereas the peptides decreased me process. The sulfur-containing compounds included two disulfides and two thiosulfates, and diminished the calcification. Glutathione, however, increased the calcification. Salicylic acid and salicylamide also gave a low level of decreased calcification. Isolated mucopolysaccharide sulfates from the cartilage were subjected to the calcification experiments, and the sulfated fraction was subject to both calcification and decreased calcification by the inhibitors, whereas the unsulfated fraction was not. An explanation of the difference in effect on calcification by the α-amino acids and peptides was based on the stability of calcium complexes of the amino acids, which assisted the process of calcification, and the lower stability of peptide calcium complexes, which did not remove the calcium from the attraction of phosphate in the medium.