For a continuous map f from the real line (half-open interval [0, 1)) into itself let ent(f ) denote the supremum of topological entropies of f | K , where K runs over all compact f -invariant subsets of R ([0, 1), respectively). It is proved that if f is topologically transitive, then the best lower bound of ent(f ) is log √ 3 (log 3, respectively) and it is not attained. This solves a problem posed by Cánovas [Dyn. Syst. 24 (2009), no. 4, 473-483].