Whey proteins from non‐lactating bovine mammary secretions reduced peripheral blood mononuclear cell (PBMC) response to mitogens. Specific proteins responsible for hyporesponsiveness are unknown. This study evaluated the influence of apo‐ and ironsaturated lactoferrin (APOLF and FELF) and transferrin (APOTF and FETF), immunoglobulin G (IgG) and serum albumin (SA) at concentrations ranging from 0·02 to 2·5 mg ml‐1on PBMC response to concanavalin A. PBMC were collected from five pregnant Holstein cows during late lactation. Lactoferrin (LF), transferrin (TF) and IgG resulted in a dose‐dependent reduction of PBMC proliferation, whereas SA had no effect. Decreased PBMC response was significant at all concentrations of APOLF and FELF, at APOTF concentrations ≥ 0.3 mg ml‐1, only at 2·5 mg ml‐1FETF, and at IgG concentrations ≥ 1·25 mg ml‐1. The degree of iron saturation of LF did not influence PBMC response; however, APOTF had a greater influence than FETF. Reduced PBMC response at low levels of LF indicate that hyporesponsiveness is not due solely to an increased protein content of the culture but may be the result of specific LF regulation of mononuclear cell proliferation.