IL-8/CXCL8 plays a critical role in the trafficking and activation of neutrophils via its receptors, CXCR1 and CXCR2, in humans. CXCR1 is highly selective for IL-8, whereas CXCR2 is activated by all CXC chemokines with an ELR motif. In mice and rats, neither IL-8 nor CXCR1 is present, making it difficult to evaluate the in vivo roles of the IL-8/CXCR1 interactions. We previously demonstrated the presence of IL-8 in the guinea pig (gp), suggesting that its specific receptor CXCR1 is also present in this species. Here, we obtained two gp genomic DNA clones, clones 8 and 10, coding for the potential orthologues of CXCR1 and CXCR2, respectively. Transcripts for these genes were expressed in neutrophils, but not in macrophages. Functionally, both gp and human (h) IL-8 induced cell migration and ERK phosphorylation in HEK 293 cells expressing either receptor, whereas hGRO activated only cells expressing the clone 10 protein, confirming that clone 8 indeed coded for gpCXCR1. 125I-labeled hIL-8 bound to gpCXCR1 and addition of unlabeled hIL-8 completely abolished the binding; however, unlabeled gpIL-8 failed to compete against 125I-labeled hIL-8, strongly suggesting that the avidity of hIL-8 to gpCXCR1 is higher than that of gpIL-8. Identification and characterization of CXCR1 in the guinea pig will allow us to use this small animal model to evaluate the role of the IL-8/CXCR1 interactions and to examine the efficacy of CXCR1 antagonists in vivo.