This note contains a derivation of a formally exact relation in the field theory between the elements of theS matrix corresponding to a given number of particles. This relation is similar in form to Low’s equation (1), but all elements of theS matrix associated with different numbers of particles but one distinguished are here eliminated (+). The derivation of this relation is based on the equation for a distinguished component of the state vector corresponding to a given number of particles. Low’s general equation obtained here contains the interaction operator of the given number of particles. So far, this operator can be evaluated practically only by expanding it in the coupling constant.