Can the hidden mass be negative?

A consequence of modified nonrelativistic dynamics (MOND) which is incompatible with the hidden mass hypothesis (HMH) is discussed. If MOND is basically correct, there are mass configurations which, when probed dynamically using Newtonian dynamics will appear to contain negative hidden mass, so that the deduced mass density in such systems is, in some regions of space, smaller than the actual density. Generalities concerning the possible appearance of negative hidden mass are discussed, and a general procedure for determining gravitational masses and the conditions for the generation of negative phantom masses is described. Actual configurations where negative phantom mass is expected by MOND are presented, and prospects for detecting such negative densities are discussed. 16 references