Peptides that bind to the interleukin-1 type I receptor (IL-1RtI) can be used to assay the amount of IL-1R, or an IL-1R agonist or antagonist, in a sample and comprise a sequence of amino acids selected from the group consisting of (1) WXXXGZ.sub.1 W where Z.sub.1 is L, I, A, or Q; (2) XXQZ.sub.5 YZ.sub.6 XX where Z.sub.5 is P or Aze where Aze is azetidine; and Z.sub.6 is S, A, V, or L; and (3) Z.sub.23 NZ.sub.24 SZ.sub.25 Z.sub.26 Z.sub.27 Z.sub.28 Z.sub.29 Z.sub.30 L where Z.sub.23 is D or Y; Z.sub.24 is D or S; Z.sub.25 is S or W; Z.sub.26 is S or Y; Z.sub.27 is D or V; Z.sub.28 is S or W; Z.sub.29 is F or L; and Z.sub.30 is D or L; and where each amino acid is indicated by standard one letter abbreviation; and each X can be selected from any one of the 20 genetically coded L-amino acids or the stereoisomeric D-amino acids. Also provided are peptides which bind to the IL-1RtI, which are 11 to 40 amino acids in length, which comprise the core sequence of amino acids: Z.sub.31 XWZ.sub.32 Z.sub.33 Z.sub.34 Z.sub.35 Z.sub.36 QZ.sub.37 Z.sub.38 where each letter represents the standard one letter abbreviation for an amino acid or an analog thereof; X is selected from the group of natural or unnatural amino acids; Z.sub.37 is a natural or unnatural cyclic amino acid; Z.sub.31 is selected from phenylalanine and acetylated phenylalanine; Z.sub.32 is a natural or unnatural amino acid; Z.sub.33 is selected from proline and pipecolic acid; Z.sub.34 is selected from glycine, d-alanine, d-valine, sarcosine and aminoisobutyric acid; Z.sub.35 is a natural or unnatural amino acid and Z.sub.36 is selected from tyrosine, phosphotyrosine, phenylalanine and tryptophan; and Z.sub.38 is selected from tyrosinamide and substituted tyrosinamide.