Formylated peptides are potent stimulants of polymorphonuclear neutrophilic leukocyte (PMN) migration from species such as humans and rabbits. Interestingly, PMNs from dogs, cats, pigs and cows have been reported as refractory to N-formyl-l-methionyl-l-leucyl-l-phenylalanine (FMLP) and generally are believed not to express formylpeptide receptors. Formylpeptides are a major component of conditioned media from E. coli cultures and believed to be a significant element in inflammatory responses elicited by E. coli. Our studies have found that E. coli filtrate was a potent stimulant of dog PMN migration. Inhibition of migration to E. coli filtrates by the antagonist t-botyloxycarbonyl-l-methionyl-l-leucyl-l-phenylalanine (t-boc-MLP) demonstrated that the migration was mediated through the formylated peptide receptor. Migration in response to peptides with higher affinity for the formylpeptide receptor than FMLP was further evidence for these receptors on the dog PMN. PMNs from dogs migrated in response to FMLP at high concentrations (100 microM); however, pretreatment with phorbol myristate acetate resulted in increased migration of dog PMNs in response to concentrations of FMLP as low as 1 pM. These results demonstrate that dog PMNs are responsive to formylpeptides and that these responses can be up-regulated by PMA. Thus PMNs from a species previously thought incapable of responding to formylpeptides can respond to formylpeptide analogs with high affinity for the receptor as well as be primed for enhanced migration to FMLP by PMA.