A Banach ∗-algebra is called
symmetric, if the spectra of elements of the form a∗a contain only nonnegative real
numbers. Symmetric Banach ∗-algebras have a series of important properties,
especially with respect to their representation theories. Here it is proved that
lensoring with finite dimensional matrix rings preserves symmetry. As an application
it is shown that the category of locally compact groups with symmetric L1-a1gebras
is closed under finite extensions.