A pilot scale study has been made of the concept of adding fibre-based strength agents (fines enriched (FE)-pulp or highly refined (HR)-pulp) in a board middle ply containing chemithermomechanical bleached pulp (CTMP) in order to increase bending stiffness of the board while maintaining Z-strength. It has been demonstrated that the bending stiffness of a sheet consisting of a top ply and a CTMP based middle ply could be improved by increasing the CTMP fraction and preventing Z-strength loss via addition of a fibre based strength agent. Compared with the reference pulp, both Z-strength and bulk increased for three of the compositions, namely 65% CTMP with 5% strength agent of either FE or HR type and 85% CTMP with 10% HR-pulp. FE-pulp was found to be more efficient than HR-pulp concerning bending stiffness improvement. While the highly-refined fibres of the strength agents had a negative effect on the drainage resistance and press dryness, an increased share of CTMP increased the press dryness linearly. FE-pulp and HR-pulp had the same impact on press dryness. Press solids could be improved by approximately 2% without significantly reducing the bulk by increasing press loads.