In early weaned calves, rumen development resulting in elevated concentrations of blood β‐hydroxybutyrate (BHBA), or stress resulting in elevated concentrations of blood cortisol, may be deleterious to cell‐mediated immunity and overall calf health. To further define the influence of specific factors generated from these physiological events during early weaning of dairy calves on the immune response, this study evaluated the influence of increasing concentrations of BHBA (0.63–10 mM), 1,3‐butanediol (0.63–10 mg ml‐1) and cortisol (12.1–194 ng ml‐1) on responses of peripheral blood mononuclear cells (PBMCs; 5 calves, 11 weeks of age) to concanavalin A (Con A), phytohemagglutinin (PHA) and pokeweed mitogen (PWM) in vitro. Responses of PBMCs to PHA and PWM were unaffected by BHBA, while BHBA significantly reduced PBMC blastogenesis in response to Con A. However, in the presence of increasing BHBA, there was a slight but significant linear decline of PBMC blastogenesis in response to PHA. Cortisol significantly reduced PBMC blastogenesis in response to all mitogens, although inhibition did not increase above 24–3 ng ml‐1. Increasing concentrations of 1,3‐butanediol caused a significant linear reduction in PBMC blastogenesis in response to all mitogens. These results suggest that elevated BHBA concurrent with rumen development in dairy calves near weaning has little influence on mononuclear cell blastogenesis, whereas elevated cortisol concentrations may increase susceptibility of dairy calves to disease.