The magnetic properties of ThCr2Si2-type structure CaMn2Ge2 and BaMn2Ge2 compounds have been investigated by neutron diffraction experiments. In the whole temperature range studied (2-270 K), both compounds are purely collinear antiferromagnets (not detected by bulk magnetometric measurements) characterized by a stacking of antiferromagnetic (001) Mn planes. This peculiar Mn-sublattice magnetic behaviour seems to be related to the valency of the large metal. At 2 K, the total Mn moments are about 2.7 mu(B) and 3.6 mu(B) for CaMn2Ge2 and BaMn2Ge2, respectively. The results are compared with those of closely related RMnSi and RMnGe compounds and the isotypic alkali-metal manganese pnictides. The magnetic properties of the ThCr2Si2-type structure RMn2X2 (X-Si, Ge) compounds are discussed.