The ability of specific bovine Ig isotypes to enhance phagocytosis of Staphylococcus aureus by polymorphonuclear neutrophils was studied. Polymorphonuclear neutrophils were isolated from the blood of 14 lactating Holstein cows. Antibodies against S. aureus M10 were produced by two Holstein cows immunized via intramuscular injections and injections in the area of the supramammary lymph node with M10 emulsified in dextran sulfate. The IgG1, IgG2, and IgM were prepared from immune sera. Fluorescein-labeled, formalin-killed S. aureus M10 were opsonized with the respective isotypes prior to incubation with polymorphonuclear neutrophils. Percentage of polymorphonuclear neutrophils phagocytosing averaged 37.4, 1.1, 15.9, and 9.4% for immune sera, IgG1, IgG2, and IgM, using a M10: polymorphonuclear neutrophils ratio of 10:1; and 77.1, 1.8, 32.1, and 57.9 using a 40:1 ratio. When IgG1 was incubated with either IgG2 or IgM, phagocytosis was reduced to 10.0 and 5.0%, respectively, using the 10:1 ratio and 24.2 and 44.7%, respectively, using the 40:1 ratio. Significant variation occurred among cows in the ability of polymorphonuclear neutrophils to undergo phagocytosis independent of isotype and S. aureus M10: polymorphonuclear neutrophil ratio. These data show that IgG2 and IgM are opsonic for bovine polymorphonuclear neutrophils and that IgG1 inhibits the activity of both. These results will be helpful to determine immunization protocols to solicit synthesis of bovine IgM and IgG2 specific for S. aureus.