20 ILCS 505/9.7

If a responsible parent or guardian fails or refuses to make parental payments for care and training of their children, or contributes less than the amount indicated by the determination, the Department shall take action to enforce support in accordance with Section 9.8 of this Act. An annual interest rate equal to the prime commercial rate of interest plus 3% will be assessed and payable on all amounts more than 60 days past due. For the purposes of this Section, "prime commercial rate" means such prime rate as from time to time is publicly announced by the largest commercial banking institution located in this State, measured in terms of total assets.