CLA intake in exclusively breast-fed infants is close to levels found to have physiological effects in animals. However, in the majority of studies mixtures of CLA isomers have been used and the independent effects of the major CLA isomer in human milk, cis-9,trans-11 CLA, at the intake level in exclusively breast-fed infants have hardly been studied. We therefore studied the effects of cis-9,trans-11 CLA on plasma lipids and glucose, immune function, and bone metabolism in growing rats. Thirty male Sprague-Dawley rats (n = 10/group) were fed either 20 mg/kg/d cis-9,trans-11 CLA and 20 mg/kg/d sunflower oil (CLA20), 40 mg/kg/d cis-9,trans-11 CLA (CLA40), or 40 mg/kg/d sunflower oil (placebo) for 8 wk. No significant differences between groups were found in plasma lipids, glucose, insulin, C-reactive protein, or lipid peroxidation. Liver fat content was lowest in the CLA20 group. In vitro interleukin 2 (IL-2) production increased, and tumor necrosis factor alpha, IL-1beta, prostaglandin E2, and leukotriene B4 production decreased in the CLA20 group. No differences between groups were detected in IL-4, IL-6, or interferon gamma production, plasma osteocalcin, insulin-like growth factor, or urinary deoxypyridinoline crosslinks. Plasma tartrate-resistant acid phosphatase 5b activity was significantly increased in the CLA40 group. The results indicate anti-inflammatory effects and enhanced T-cell function for the CLA20 group. No adverse effects were seen in the CLA20 group, whereas indications of increased bone resorption rate were observed in the CLA40 group.