diff --git a/Generators/AtlasHepMC/CMakeLists.txt b/Generators/AtlasHepMC/CMakeLists.txt
index f3e29e5291e4635de0f38b920b345ea3ea8c318b..6db36811bdd5dc0dad750b634dee6b8eeaaeb5a8 100644
--- a/Generators/AtlasHepMC/CMakeLists.txt
+++ b/Generators/AtlasHepMC/CMakeLists.txt
@@ -7,11 +7,16 @@ find_package( HepMC COMPONENTS HepMC HepMCfio )
 find_package( hepmc3 COMPONENTS HepMC3 HepMC3search )
 
 if (HEPMC3_USE)
+
+# Tell the user what's happening.
+message( STATUS "Using HepMC3 for AtlasHepMC" )
+
 # Component(s) in the package.
 atlas_add_library( AtlasHepMCLib
    AtlasHepMC/*.h
    INTERFACE
    PUBLIC_HEADERS AtlasHepMC
+   DEFINITIONS -DHEPMC3=1
    INCLUDE_DIRS  ${HEPMC3_INCLUDE_DIRS}
    LINK_LIBRARIES ${HEPMC3_HepMC3_LIBRARY}  )
 
@@ -19,17 +24,23 @@ atlas_add_library( AtlasHepMCfioLib
    AtlasHepMC/*.h
    INTERFACE
    PUBLIC_HEADERS AtlasHepMC
-   INCLUDE_DIRS  ${HEPMC3_INCLUDE_DIRS} 
+   DEFINITIONS -DHEPMC3=1
+   INCLUDE_DIRS  ${HEPMC3_INCLUDE_DIRS}
    )
 
 atlas_add_library( AtlasHepMCsearchLib
    AtlasHepMC/*.h
    INTERFACE
    PUBLIC_HEADERS AtlasHepMC
+   DEFINITIONS -DHEPMC3=1
    INCLUDE_DIRS  ${HEPMC3_INCLUDE_DIRS}
    LINK_LIBRARIES ${HEPMC3_HepMC3search_LIBRARY}  )
 
 else()
+
+# Tell the user what's happening.
+message( STATUS "Using HepMC for AtlasHepMC" )
+
 # Component(s) in the package.
 atlas_add_library( AtlasHepMCLib
    AtlasHepMC/*.h
@@ -42,7 +53,7 @@ atlas_add_library( AtlasHepMCsearchLib
    AtlasHepMC/*.h
    INTERFACE
    PUBLIC_HEADERS AtlasHepMC
-   INCLUDE_DIRS ${HEPMC_INCLUDE_DIRS}  
+   INCLUDE_DIRS ${HEPMC_INCLUDE_DIRS}
    )
 atlas_add_library( AtlasHepMCfioLib
    AtlasHepMC/*.h