Mo. Rev. Stat. § 540.060

Before a grand juror is sworn, any person held to answer a criminal charge may object to the competency of the grand juror on the ground that the grand juror is the prosecutor or complainant upon any charge against such person, or that the grand juror is a witness on the part of the prosecutor and has been summoned or bound in a recognizance as such. If such objection is established, the person so challenged shall be excused.