Three trials were conducted to evaluate the effect of moniliformin (M) on performance and immune function in chicks. Day-old chicks were randomly assigned to four dietary treatments (0, 50, 75, or 100 mg M/kg diet). In Trial 1, chicks were placed on treatments for 3 wk and were injected intravenously with 4.6 x 10(6) Escherichia coli on Day 21. Blood samples were collected at 60, 120, and 180 min after inoculation, and liver, spleen, and lung were collected at 180 min postinjection. Compared with control chicks, chicks fed 75 and 100 mg M/ kg diet had higher (P < 0.05) numbers of E. coli colonies in the circulation, liver, and spleen. In Trial 2, chicks were placed on diets for 4 wk and were injected with 0.5 mL Newcastle disease virus (NDV) vaccine intramuscularly on Weeks 2 and 3 of the experiment. The primary and secondary anti-NDV antibody titers were measured 7 d after each injection. Chicks fed 100 mg M/kg diet had lower (P < 0.05) secondary antibody titers than did control chicks. In Trial 3, lymphocyte proliferation in chicks exposed to M in vivo and in vitro was determined. Results of the in vivo study showed that cell proliferation in response to mitogens from control- and M-fed chicks did not differ (P > 0.05). For the in vitro study, lymphocyte proliferation decreased linearly (P < 0.01) with increased concentrations of M. In all three trials, chicks fed 100 mg M/kg diet had lower (P < 0.05) feed intake and weight gain than did control chicks. Data from the current study suggested that M decreased performance and immune response in chicks at the level of 75 mg/kg diet.