Pretreatment with rmIL-12 attenuates the IFN-γ response following subsequent rmIL-12 administration. Mice received a single injection of either rmIL-12 (0.5 μg) or vehicle 7 days before daily rmIL-12 administration (days 0 to 5). Serum was collected by cardiac puncture during the period of daily rmIL-12 administration and IFN-γ measured by standard ELISA techniques. Individual mouse IFN-γ measurements are shown. Comparable results were seen in a separate experiment.