This study was undertaken to investigate the growth performance and immune responses of broiler chicks fed diets supplemented with conjugated linoleic acid (CLA). Two hundred and forty day-old Arbor Acre male broiler chicks were randomly allotted into four dietary treatments with different inclusion levels of CLA (0, 2·5, 5·0 or 10·0g pure CLA/kg) for 6 weeks. Growth performance, lysozyme activity, peripheral blood mononuclear cell (PBMC) proliferation, prostaglandin E2 (PGE2) synthesis and antibody production were investigated. There were no significant differences in growth performance among treatments (P>0·05). Chicks fed 10·0g CLA/kg diet produced 40% and 49% more lysozyme activity in serum and spleen than the control group at 21d of age (P<0·05). Dietary CLA enhanced the PBMC proliferation in response to concanavalin A at the age of 21 and 42d (P<0·05). Systemic and peripheral blood lymphocytic synthesis of PGE2 in chicks fed 10·0g CLA/kg diet was significantly decreased by 57% and 42% compared to chicks fed control diet (P<0·05). Antibody production to sheep red blood cell and bovine serum albumin were elevated in either 2·5 or 10·0g CLA/kg dietary treatments (P<0·05). The results indicated dietary CLA could enhance the immune response in broiler chicks, but did not alter the growth performance.