Peptides or small molecules that can block the interaction of the integrin Mac-1 with its receptor, intercellular adhesion molecule-1 (ICAM-1), have not previously been developed. We studied this interaction by measuring the adherence of ICAM-1-expressing Chinese hamster ovary (CHO) cells to immobilized, purified Mac-1. Nucleotide sequence information was obtained for the complementarity determining regions (CDRs) of three antibodies (44aacb, MY904, and 118.1) shown to block Mac-1-mediated cell adherence. Peptides were synthesized based on the predicted amino acid sequences of the CDRs and tested for the ability to block cell adhesion to Mac-1. Peptides derived from CDR1 of 44aacb, CDR2 of 118.1, and CDRs 1 and 3 of MY904 heavy chains were found to possess blocking activity at 10-100 muM. This may indicate that one or two CDRs contribute disproportionately to the antibody binding affinity. The binding of ligands to Mac-1 has been shown to require a region of the alpha-chain known as the I- or A-domain. We have recombinantly produced Mac-1 I-domain, and show that it is also capable of supporting the adherence of ICAM-1-expressing CHO cells. The adherence of ICAM-1-CHO cells to the I-domain is inhibited by 44aacb and 118.1 and by the CDR peptides from 44aacb and 118.1. By using phage display of peptide libraries based on the 118.1 CDR peptide with five residues randomized, we were able to identify a novel peptide inhibitor of Mac-1 with substitutions at all five positions. These peptides provide lead structures for development of Mac-1 antagonists.