Bovine monocytes were isolated from the peripheral blood of cattle by adherence of peripheral blood mononuclear cells to plastic. Three in vitro methods were modified to evaluate bovine monocyte function. These methods were: (1) ingestion of 125I-iododeoxyuridine labeled Staphylococcus aureus; (2) antibody-dependent cell-mediated cytotoxicity; (3) luminol-dependent and native chemiluminescence. Description of monocyte isolation and assay development is discussed in the text. Assays to evaluate monocyte function are useful for assessing immune status of the animal.