Given a set Γ of modal formulas of the form γ(x,p), where x occurs positively in γ, the language L#(Γ) is obtained by adding to the language of polymodal logic connectives #γ, γ in Γ. Each term #γ is meant to be interpreted as the least fixed point of the functional interpretation of the term γ(x). Given such a Γ, we construct an axiom system K#(Γ) which is sound and complete w.r.t. the concrete interpretation of the language L#(Γ) on Kripke frames. If Γ is finite, then K#(Γ) is a finite set of axioms and inference rules.