Bovine mammary gland mononuclear cells (MGMC) and peripheral blood mononuclear cells (PBMC) were isolated during the non‐lactating period to determine if recombinant bovine somatotropin (rbST) enhanced mitogen‐stimulated mononuclear cell proliferation and/or overcame suppressive effects of mammary secretions on mononuclear cell proliferation in vitro. MGMC were hyporesponsive to mitogenic stimulation compared with PBMC. Mammary secretions markedly suppressed the MGMC and PBMC response to mitogens. rbST had no effect on mononuclear cell proliferation in the presence or absence of mitogens. Similarly, rbST did not reverse the suppression of mononuclear cell proliferation by mammary secretions. Under the conditions of this study, rbST had no direct effect on the proliferation of mature bovine mononuclear cells.