The effect of Escherichia coli lipopolysaccharide (LPS) on the expression of major histocompatibility complex (MHC) class II molecules by bovine mammary macrophages was examined. The ability of LPS-treated mammary macrophages to support antigen-specific T-cell proliferation, as a measure of their antigen presentation ability, was also evaluated. For this purpose, control and LPS-treated macrophages were pulsed with heat-killed Staphylococcus aureus and then cultured with S. aureus-sensitized T-cells. Our data show that LPS had no significant effect on the expression of MHC class II molecules on the surface of mammary macrophages. Furthermore, LPS-induced macrophages were no more active in supporting T-cell proliferation on a per cell basis than unstimulated macrophages. The lack of macrophage response to LPS with respect to expression of MHC class II molecules and the antigen presentation ability is another example of the hyporesponsive nature of macrophages isolated from the bovine mammary gland.