This study presents a novel approach to composite membranes production. Our membranes are based on polysulfone matrix and filled with inorganic hydroxyapatite nanoparticles (HAp NPs) as new green pore-forming agent. A non-classical HAp growth mechanism – oriented attachment (OA) – was utilized for the NPs' size control. The rod-shaped HAp NPs were investigated using XRD, WPPM, FTIR and TEM analyses complemented by quantum-chemical calculations to estimate the role of inorganic ion regulators in surface blocking and coalescence course. The HAp nanofillers with different sizes and aspect ratios were incorporated into polysulfone membranes, with structural and morphological characterization via XRD, ATR-FTIR, SEM and AFM methods. Filler size dictated the morphology of obtained defect-free membranes: larger filler increased surface roughness, while smaller HAp sample induced irregular pore formation. Ultrafiltration performance of composite membranes was evaluated using water and model protein solution with comparison to polyethylene glycol 2000 plasticized blank membrane. HAp-modified membranes exhibited a substantial increase in pure water flux versus blanks, while protein flux rose moderately, indicating enhanced desirable protein retention. It was established that the use of HAp nanofillers of different sizes allows to control the transport properties of ultrafiltration membranes and achieve greater separation efficiency during the purification of protein medium. Post-treatment revealed a decline in water flux recovery compared to blanks, attributed to temporary binding of BSA to HAp surface highlighting the need for optimized membrane washing protocols. These outcomes demonstrate great prospects of OA-synthesized HAp nanofillers to tailor polysulfone membranes for biomedical and food-industry applications. © 2024