The paper presents a new proof-number (PN) search algorithm, called PDS-PN. It is a two-level search (like PN2), which performs at the first level a depth-first proof-number and disproof-number search (PDS), and at the second level a best-first FIN search. Hence, PDS-PN selectively exploits the power of both PN2 and PDS. Experiments in the domain of Lines of Action are performed. They show that within an acceptable time frame PDS-PN is more effective for really hard endgame positions than alphabeta and any other PN-search algorithm.