The statistical mechanical theory for the helix-to-random-coil transition in two-chain coiled coils is applied to extant data for two synthetic coiled-coil polypeptides. These peptides have the primary structure K(LEALEGK),, in which n = 4,5. This repeating heptet sequence mimics the pattern of hydrophobic, acidic, and basic residues characteristic of the 284-residue tropomyosin molecule, the prototypical coiled-coil protein. Theoretical calculations for single chains show that such model peptides cannot be directly compared to proteins like tropomyosin because of differences in chain length (29 and 36 residues vs 284) and in intrachain interactions, the latter caused by the differences in amino acid composition and sequence between protein and model. Application of the theory to extant data on the two synthetic peptides provides a semiquantitative fit and results in an assessment of the interhelix interaction in the model peptides. The value obtained, -2000 cal . (mol of turn pairs)-', is four to five times larger than has been obtained for tropomyosin. This probably is a result of greater regularity in the structure of the synthetics and of the exclusive presence of leucine in the hydrophobic interface. The theory employed here insists that this powerful interhelix interaction in the synthetic is the principal reason that such short chains can be so highly helical at moderate and low temperatures. Theory predicts, indeed, that a tropomyosin-length chain with a sequence homologous to these synthetics would be completely thermally stable in the entire temperature range accessible in aqueous solutions. Theory also predicts a much more pronounced effect of concentration on the 29-and 36-residue synthetic polymers than is predicted or observed in the case of tropomyosin, and it also predicts a pronounced stabilizing effect of pH-reduction on the thermal curves. On the last two points, sufficient data are not yet available with which to test the theory. *To whom reprint requests should be sent.