diff --git a/tests/regression_tests/plot/test.py b/tests/regression_tests/plot/test.py
index bfaef019c1..a6f7306a0e 100644
--- a/tests/regression_tests/plot/test.py
+++ b/tests/regression_tests/plot/test.py
@@ -42,10 +42,10 @@ class PlotTestHarness(TestHarness):
                 # Add voxel data to results
                 with h5py.File(fname, 'r') as fh:
                     outstr += fh.attrs['filetype']
-                    outstr += fh.attrs['num_voxels'].tostring()
-                    outstr += fh.attrs['lower_left'].tostring()
-                    outstr += fh.attrs['voxel_width'].tostring()
-                    outstr += fh['data'].value.tostring()
+                    outstr += fh.attrs['num_voxels'].tobytes()
+                    outstr += fh.attrs['lower_left'].tobytes()
+                    outstr += fh.attrs['voxel_width'].tobytes()
+                    outstr += fh['data'][()].tobytes()
 
         # Hash the information and return.
         sha512 = hashlib.sha512()
diff --git a/tests/regression_tests/plot_overlaps/test.py b/tests/regression_tests/plot_overlaps/test.py
index d8c0943cf5..62236081b3 100644
--- a/tests/regression_tests/plot_overlaps/test.py
+++ b/tests/regression_tests/plot_overlaps/test.py
@@ -42,10 +42,10 @@ class PlotTestHarness(TestHarness):
                 # Add voxel data to results
                 with h5py.File(fname, 'r') as fh:
                     outstr += fh.attrs['filetype']
-                    outstr += fh.attrs['num_voxels'].tostring()
-                    outstr += fh.attrs['lower_left'].tostring()
-                    outstr += fh.attrs['voxel_width'].tostring()
-                    outstr += fh['data'].value.tostring()
+                    outstr += fh.attrs['num_voxels'].tobytes()
+                    outstr += fh.attrs['lower_left'].tobytes()
+                    outstr += fh.attrs['voxel_width'].tobytes()
+                    outstr += fh['data'][()].tobytes()
 
         # Hash the information and return.
         sha512 = hashlib.sha512()
diff --git a/tests/regression_tests/plot_voxel/test.py b/tests/regression_tests/plot_voxel/test.py
index d2767e2f18..3b7a6bef28 100644
--- a/tests/regression_tests/plot_voxel/test.py
+++ b/tests/regression_tests/plot_voxel/test.py
@@ -23,7 +23,7 @@ class PlotVoxelTestHarness(TestHarness):
 
         check_call(['../../../scripts/openmc-voxel-to-vtk'] +
                    glob.glob('plot_4.h5'))
-                
+
     def _test_output_created(self):
         """Make sure *.ppm has been created."""
         for fname in self._plot_names:
@@ -44,10 +44,10 @@ class PlotVoxelTestHarness(TestHarness):
                 # Add voxel data to results
                 with h5py.File(fname, 'r') as fh:
                     outstr += fh.attrs['filetype']
-                    outstr += fh.attrs['num_voxels'].tostring()
-                    outstr += fh.attrs['lower_left'].tostring()
-                    outstr += fh.attrs['voxel_width'].tostring()
-                    outstr += fh['data'].value.tostring()
+                    outstr += fh.attrs['num_voxels'].tobytes()
+                    outstr += fh.attrs['lower_left'].tobytes()
+                    outstr += fh.attrs['voxel_width'].tobytes()
+                    outstr += fh['data'][()].tobytes()
 
         # Hash the information and return.
         sha512 = hashlib.sha512()