AbstractIt has recently been observed that the symmetric case of the biconjugate gradient method renders a variational formula for the monostatic scattering amplitude stationary in the available Krylov subspace at each iteration. In this Letter I point out that the general biconjugate gradient method makes stationary the analogous variational formula for a bistatic scattering amplitude and suggest likely practical benefits accruing from this property. © 1993 John Wiley & Sons, Inc.