Governor Granholm has vetoed a bill that would have allowed adult motorcyclists to ride without helmets.
The mandatory helmet law, which dates back to the 1960s, will remain in place.
The veto disappoints a group called American Bikers Aiming Toward Education of Michigan.
The group had successfully urged both the House and Senate to pass the bill, getting the measure as close to final approval as it has ever been.
But insurance groups and health-related agencies had lobbied against the bill.
The legislation would have let riders 21 and older decide whether to wear a helmet if they met certain conditions.