Cyclolinopeptide A (CLA), a cyclic nonapeptide from linseed, possesses strong immunosuppressive and antimalarial activity along with the ability to inhibit cholate uptake into hepatocytes. The structure of the peptide was studied extensively in solution as well as in the solid state. It is postulated that both the Pro-Pro cis-amide bond and an 'edge-to-face' interaction between the aromatic rings of two adjacent Phe residues are important for biological activity. Structure-activity relationship studies of many linear and cyclic analogues of CLA suggest that the Pro-Xxx-Phe sequence and the flexibility of the peptide are important for the immunosuppressive activity.