A mechanical overspeed protection device incorporated in a microtest plate carrier used in a centrifuge rotor. A locking arm actuates into a blocking position on the microtest plates in the carrier to indicate that the carriers on the rotor have been subjected to an overspeed condition during the centrifugation run. A housing within the carrier microtest plate retains the arm in an unactivated position during operation below a specified force field. However, when the carrier experiences an overspeed condition, the locking arm is designed to have sufficient weight, so that in response to the centrifugally induced forces the arm will move out of the housing into an outward blocking position on the carrier plates. The insertion of new microtest plates in the carrier for additional centrifugation runs is prevented, since the locking arm is in its blocking position. The blocking position of the locking arm will notify the operator that other carriers should replace the ones which have been subjected to an overspeed condition.