Neutrophils play an essential role in bovine cellular host defense, and compromised leukocyte function has been linked to the development of respiratory and mucosal infections. During the host defense process, neutrophils migrate into infected tissues where they become activated, resulting in the assembly of neutrophil membrane and cytosolic proteins to form a superoxide anion-generating complex known as the NADPH oxidase. Two of the essential cytosolic components of the NADPH oxidase are p47-phox and p67-phox. Currently, only the human and murine homologs of these proteins have been sequenced. Because of the important role neutrophils play in bovine host defense, we carried out studies to clone, sequence, and express bovine p47-phox and p67-phox. Using polymerase chain reaction (PCR) cloning techniques and a bovine bone marrow cDNA library, we have cloned both of these bovine NADPH oxidase cytosolic components. Comparison of the bovine sequences with those of the human and murine homologs showed that they were highly conserved, but also revealed important information regarding key structural features of p47-phox and p67-phox, including location of putative phosphorylation sites. Functional expression of bovine p47-phox and p67-phox showed that these proteins could substitute for the human proteins in reconstituting NADPH oxidase activity in a cell-free assay system, again demonstrating the high degree of conservation between human and bovine homologs. This study greatly contributes to our understanding of the potential structural/functional regions of p47-phox and p67-phox as well as providing information that can be used to study the role of neutrophils in bovine inflammatory diseases.