We have prepared antisera in both rabbits and rats against f Met-Leu-Phe conjugated to a variety of carrier proteins. Over 40 peptides with widely varying reactivity for the neutrophil formylpeptide receptor have been tested for their ability to bind to rabbit antibody raised against fMLP10-BSA. Structure-activity studies of peptides structurally related to f Met-Leu-Phe demonstrate that the N-formyl group is mandatory for maximum antibody binding activity. Methionine in position 1 and phenylalanine in position 3 are found to confer maximum binding activity. Stereoselectivity of the antibody-combining site also has been demonstrated. Comparison of the ability of the peptides to bind to the antibody receptor with their reactivity for the neutrophil has demonstrated a strong correlation in the rank order of reactivity of the numerous synthetic peptides: for the alpha NH2-acyl group, r = 0.94; for position 1, r = 0.90; for position 2, r = 0.97; and for position 3, r = 0.78. This strong correlation is seen across species lines with both rabbit and rat antibodies. Significant differences, however, in the specificity of the antibody and neutrophil receptors are seen at the carboxyl terminus of phenylalanine, and beyond the phenylalanine ring, r = 0.29. In addition, bacterial chemotactic factor-enriched butanol extracts from Escherichia coli culture filtrates can also bind to anti-f Met-Leu-Phe, affording additional evidence for the similarity in the structure of the bacterial chemotactic factor to the synthetic chemotactic peptides.