The aim of this study was to evaluate the effect of varying dietary P on bovine immune function. Nine first- or second-lactation Holstein cows were fed diets varying in P in a 3 x 3 Latin square design. Diets were formulated to contain either low (0.34%, no supplementary P), medium (0.43%), or high (0.52%) P. All 3 diets were formulated to meet or exceed current NRC requirements for P content. Between d 21 and 26 of each period, blood samples were collected and serum inorganic P concentration, lymphocyte proliferation, and neutrophil bactericidal activity were measured. Serum P increased with increasing dietary P intake and was greatest in the first lactation compared with subsequent lactations. There was a stage of lactation-dependent increase in lymphocyte proliferation after stimulation with concanavalin A, phytohemagglutinin, or pokeweed mitogen. However, dietary P did not alter lymphocyte proliferation or neutrophil bactericidal activity in vitro. In conclusion, decreasing dietary P to reduce manure P content and the risk of P losses from farms to surface water does not have an adverse effect on the innate or cell-mediated immune responses of lactating dairy cattle.