The immunosuppressive activity of a linear peptide related to cyclolinopeptide A, Leu-Ile-Ile-Leu-Val-Pro-Pro-Phe-Phe, and its nine analogs, where each successive amino acid residue was substituted by alanine, was investigated using the plaque forming cell and delayed type hypersensitivity tests for the humoral and cellular immune response, respectively. The linear peptide was less active than its cyclic counterpart in the humoral, and equally active in the cellular immune response. All alanine analogs showed quite strong immunosuppressive potencies in the humoral immune response tested in vitro. In the in vivo humoral immune response their activities were much less differentiated than it was observed in the in vitro experiments. All alanine analogs retain the immunosuppressive activity in the cellular immune response. However, their activities are practically not differentiated, and it is impossible to decide which amino acid side chains of the parent peptide are the most important for generation of the immunosuppressive effects. CD spectra of alanine analogs show that, in comparison with the parent peptide, conformational equilibria of these peptides are shifted towards unordered conformations. This finding correspond well to the results of the biological tests which show that all alanine analogs are less biologically active than the parent peptide. The results of CD measurements suggest also that interaction of the hydrophobic residues located at the ends of the peptide chain may be of importance for stabilization of the folded structure of the investigated compounds.