In the present note we define the relation "the predicate formula _ is deductively valid in the theory _" which is analogous to the relation "the predicate formula _ is valid in the nonempty subject domain _." A method is proposed to prove statements of the type "a set _ ofpredicate formulas is areduction class" by using the property of the mentioned relation. The majority of the analysis is made in parallel for classical and constructive predicate calculus.1. Henceforth, the following concepts and notations are used. We denote by C and K respectively the classical and constructive predicate calculus without functional symbols. We consider ~,. 'lO.~ , • ' • • to be the subj ect variables of these calculuses. We will understand the cl ass i c a I (c 0 ns t r u c ti v e) th e 0 r y to be an arbitrary recursively axiomatized theory based on the classical (or constructive) predicate calculus (possibly with equality and functional symbols). We will consider ;:(.1' ~lI.' '" to be the subject variables of all such theories also. We will understand the formula (or term) to be a formula (or term) of any theory of the mentioned kind.