The mechanism for the hypo‐responsiveness of mammary gland mononuclear cells (MGMCs) to mitogen stimulation is poorly understood. It has been hypothesized that hypo‐responsiveness of MGMCs may be due to interference by lactoferrin or other whey proteins present in mammary secretions with lymphocyte proliferation stimulated by interleukin‐2 (IL‐2). To address this hypothesis, an experiment was designed to evaluate proliferation of an IL‐2‐dependent bovine cytotoxic T‐lymphocyte cell line (BT‐2) in the presence of apo‐ and iron‐saturated 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‐1. The lowest concentration of APOLF and FELF increased BT‐2 cell proliferation. Conversely, increasing concentrations of LF significantly inhibited BT‐2 cell proliferation. Only the highest concentrations of APOTF, FETF and IgG negatively influenced BT‐2 cell proliferation. Serum albumin had no effect. These data suggest that LF has a dose‐dependent bimodal influence on a bovine IL‐2‐dependent cytotoxic T‐lymphocyte cell line, and high concentrations of LF and other mammary secretion whey proteins may be associated with MGMC hypo‐responsiveness during the non‐lactating period, possibly through interference with IL‐2‐stimulated lymphocyte proliferation.