Page "Negative mass" Paragraph 12

On the other hand, if we have a small object with equal inertial and passive gravitational masses falling in the gravitational field of an object with negative active gravitational mass ( a small mass dropped above a negative-mass planet, say ), then canceling out and would indicate that the acceleration a of the small object is proportional to the negative active gravitational mass of the object creating the gravitational field, so the small object would actually accelerate away from the negative-mass object rather than towards it ( and this is true regardless of whether the small object's inertial and passive gravitational masses were both positive or both negative ).

So, as long as inertial mass and gravitational mass are always equal as required by the equivalence principle, positive active gravitational mass would be universally attractive ( both negative-mass and positive-mass objects would be pulled towards an object with positive active gravitational mass ), while negative active gravitational mass would be universally repulsive ( both negative-mass and positive-mass objects would be pushed away ).