We have reported previously that bovine neutrophils are unable to kill the bovine respiratory pathogen "Haemophilus somnus." In the present study we expanded our efforts and examined the interaction of bovine mononuclear phagocytes with this important veterinary pathogen. Bovine alveolar macrophages and blood monocytes ingested but did not kill opsonized "Haemophilus somnus" in vitro, whereas these same cells ingested and killed opsonized Escherichia coli. Because this suggested that "H. somnus" was a facultative intracellular pathogen, we developed an assay to monitor the intracellular fate of ingested "H. somnus" within bovine monocytes. Our results indicated that ingested "H. somnus" multiplied within bovine monocytes (1- to 2-log10 increase in 4 h); equivalent intracellular growth was noted for both a laboratory strain and a recent field isolate of "H. somnus." Bovine monocytes killed ingested E. coli (1- to 2-log10 decrease in 4 h) under the same assay conditions that were used to follow intracellular growth of "H. somnus," thus indicating that the assay conditions did not induce a generalized defect in monocyte antibacterial activity. Light and electron microscopic examination of "H. somnus"-infected monocytes confirmed that intracellular growth had occurred. We did not observe an obvious correlation between the release of superoxide anion from bovine mononuclear phagocytes that had ingested opsonized "H. somnus" and E. coli and the subsequent intracellular survival of the bacteria. The results of this study suggest that infected mononuclear phagocytes sustain "H. somnus" infections in cattle and thus contribute to the subacute to chronic clinical course that has been reported.