The recent introduction of iridium (Ir) into the catalytic converter has resulted in an increase of Ir levels into the environment, especially ambient air and soil. These observations suggested the need to evaluate potential toxicity due to Ir exposure. Since Ir compounds have been previously shown to cause immune sensitization in humans, the effects of Ir via drinking water for 90 days was assessed in adult female Wistar rats with respect to selected immune parameters. The Ir exposure induced dose-dependent decrease (p < .01) in T helper 1 (Th1) cytokines and increase (p < .001) in a T helper 2 (Th2) cytokine. The findings show that the Ir exposure affects an immune imbalance with a skewing toward a Th2 bias, a risk factor for asthma.