Mich. Comp. Laws § 691.1413: Damage arising out of performance of proprietary function.

Sec. 13.

The immunity of the governmental agency shall not apply to actions to recover for bodily injury or property damage arising out of the performance of a proprietary function as defined in this section. Proprietary function shall mean any activity which is conducted primarily for the purpose of producing a pecuniary profit for the governmental agency, excluding, however, any activity normally supported by taxes or fees. No action shall be brought against the governmental agency for injury or property damage arising out of the operation of proprietary function, except for injury or loss suffered on or after July 1, 1965.