Fifty-one Jersey bull calves (5 +/- 1 d old) were assigned to 1 of 3 milk replacers to determine the effects of increasing doses of n-3 fatty acids from fish oil on immunocompetence and health. All calves were fed a 22.5% crude protein and 18% lipid industry standard milk replacer supplemented with an additional 2% fatty acids. The 3 treatments differed only in the supplemental lipid source and included a 3:1 mix of corn and canola oils; a 1:1 blend of fish oil and the 3:1 mix of corn and canola oils; and fish oil only. All treatments were supplemented with 150 mg of vitamin E/kg of milk replacer. Body weight, height at withers, and length between withers and pins were measured weekly. Fecal and respiratory scores were recorded multiple times daily, and peripheral blood samples were collected on d 0, 7, 14, 21, and 42 for hematologic and metabolic analyses. Immunocompetence of calves was evaluated in vitro by the ability of neutrophils and monocytes to phagocytose Escherichia coli and produce an oxidative burst and in vivo as the change in ear thickness after an intradermal injection of phytohemagglutinin-P, and the primary and secondary humoral responses to ovalbumin. Production and health parameters were unaffected by treatments. There were no significant treatment or treatment x time effects on phagocytosis; however, there was a significant quadratic response for the percentage of neutrophils producing an oxidative burst. Fish oil did not affect the change in ear thickness in response to phytohemagglutinin-P. There was also no treatment effect on the primary IgG humoral response to ovalbumin, but there was a significant quadratic treatment effect on the secondary IgG response. Adding fish oil to milk replacer altered various immune responses, and the effect was dose-dependent; however, neither production performance nor indices of health were altered when fish oil replaced 5 to 10% of the fatty acids in milk replacer.