Abstract: PL37 (RAARISLGPRCIKAFTE [SEQ ID NO: 2]) is an Antisense Homology Box peptide composed of amino acids 37 to 53 of C5a-anaphylatoxin. Complementary peptides, ASGAPAPGPAGPLRPMF (Pep-A [SEQ ID NO: 1]) and ASTAPARAGLPRLPKFF (Pep-B [SEQ ID NO: 3]) were designed and characterized. Pep-A bound to PL37 and to C5a with very slow dissociation, whereas Pep-B failed to bind at all. C5a was inactivated by 7 nM or more of Pep-A and this concentration of Pep-A inhibited induction of intracellular Ca++ influx in neutrophils. Patch clamp studies also showed the effectiveness of Pep-A in C5a-receptor-expressing neuroblastoma cells. Pep-A administration prevented rats from C5a-mediated rapid lethal shock. A-Pep-A (Pep-A acetylated with alanine at the amino-terminus) was more stable in vivo and showed stronger inhibition of inflammatory reactions in mice and rats. Chemical modification of Pep-A (e.g.