Let {xj,xj′} be a complete
biorthogonal sequence with each xj in a Banach space X and each xj′ in the
conjugate space x∗. A study is made of the summability properties of the series
Σjxj′(x)xj and ΣJxj′(x)x′(xj) as x ranges over X and x′ over x∗. Conditions are
given for matrix and abstract methods of series summability for these series in terms
of certain sequence spaces which arise naturally in connection with the biorthogonal
sequence.