Plastic materials can carry  memory of past mechanical treatment in the form of internal stress.
  We introduce a natural definition of the vorticity of internal stress
in simple two-dimensional models of elasto-plastic　fluids, which generates the internal stress.
  We demonstrate how the internal stress is self-organized after external forcings, and how it modifies, once it is freezed, the plastic behavior.
  Relation to the memory effect of glass transition (Miyamoto et al.) will also be adressed.