Polarized neutron diffraction has been used to determine the manganesemagnetic moments aligned by magnetic fields applied parallel to the [001]axis in the antiferromagnetic phases of Mn5Si3. It has been foundthat in the two antiferromagnetic phases which exist below TN themagnetic moments on the manganese sites which can be aligned by a magneticfield all have the same order of magnitude, although in the antiferromagneticstructures the ordered moments are very different. The transition, on cooling,from the AF2 to the AF1 phase is accompanied by a sharp decrease in thesusceptibilities of all the sites. This uniform decrease is not consistent withdisordered local moments on the sites which do not have ordered moments, butcan be understood using the same criteria for stability of Mn momentsas have been invoked for manganese rare-earth compounds (Ballou R,Lacroix C and Nunez Regueiro M D 1991 Phys. Rev. Lett. 66 1910).