A probability distribution function, cast in the representation of Hendrickson & Lattman [Acta Cryst. (1970), B26, 136-143], has been derived for the phase information from measurements of multiwavelength anomalous diffraction (MAD). This probability function readily permits one to determine figure-of-merit weights similar to those used in isomorphous replacement, and the coefficients that characterize this distribution function facilitate the combining of MAD phasing with results from other sources of phase information. This probability representation was derived in the course of a structural analysis of selenobiotinyl streptavidin from MAD data and applications have also been made in the structure determinations of interleukin-1 alpha and a drug complex with brominated DNA.