diff --git a/src/NETGENPlugin/NETGENPlugin_Hypothesis.hxx b/src/NETGENPlugin/NETGENPlugin_Hypothesis.hxx
index e1f0dc1..0d9da59 100644
--- a/src/NETGENPlugin/NETGENPlugin_Hypothesis.hxx
+++ b/src/NETGENPlugin/NETGENPlugin_Hypothesis.hxx
@@ -39,8 +39,6 @@
 //  Parameters for work of NETGEN
 //
 
-using namespace std;
-
 class NETGENPLUGIN_EXPORT NETGENPlugin_Hypothesis: public SMESH_Hypothesis
 {
 public:
diff --git a/src/NETGENPlugin/NETGENPlugin_SimpleHypothesis_2D.hxx b/src/NETGENPlugin/NETGENPlugin_SimpleHypothesis_2D.hxx
index d114c02..bcd30da 100644
--- a/src/NETGENPlugin/NETGENPlugin_SimpleHypothesis_2D.hxx
+++ b/src/NETGENPlugin/NETGENPlugin_SimpleHypothesis_2D.hxx
@@ -34,8 +34,6 @@
 //  Simplified parameters of NETGEN
 //
 
-using namespace std;
-
 class NETGENPLUGIN_EXPORT NETGENPlugin_SimpleHypothesis_2D: public SMESH_Hypothesis
 {
 public:
@@ -88,8 +86,8 @@ public:
   bool GetAllowQuadrangles() const;
 
   // Persistence
-  virtual ostream & SaveTo(ostream & save);
-  virtual istream & LoadFrom(istream & load);
+  virtual std::ostream & SaveTo(std::ostream & save);
+  virtual std::istream & LoadFrom(std::istream & load);
 
   /*!
    * \brief Set parameters by mesh
diff --git a/src/NETGENPlugin/NETGENPlugin_SimpleHypothesis_3D.hxx b/src/NETGENPlugin/NETGENPlugin_SimpleHypothesis_3D.hxx
index cb5dc5a..2c2be03 100644
--- a/src/NETGENPlugin/NETGENPlugin_SimpleHypothesis_3D.hxx
+++ b/src/NETGENPlugin/NETGENPlugin_SimpleHypothesis_3D.hxx
@@ -34,8 +34,6 @@
 //  Simplified parameters of NETGEN
 //
 
-using namespace std;
-
 class NETGENPLUGIN_EXPORT NETGENPlugin_SimpleHypothesis_3D: public NETGENPlugin_SimpleHypothesis_2D
 {
 public:
@@ -48,8 +46,8 @@ public:
   double GetMaxElementVolume() const { return _volume; }
 
   // Persistence
-  virtual ostream & SaveTo(ostream & save);
-  virtual istream & LoadFrom(istream & load);
+  virtual std::ostream & SaveTo(std::ostream & save);
+  virtual std::istream & LoadFrom(std::istream & load);
 
   /*!
    * \brief Set parameters by mesh