High-density polyethylene-hydroxyapatite (HDPE-HA) composites with different filler content (5-20 %) were synthesized by in situ ethylene polymerization. Good filler dispersion was observed without formation of agglomerates. Osteoblast cell behavior in HDPE-HA composites was evaluated in terms of adhesion, alkaline phosphatase activity and proliferation. Fluorescence and scanning electron microscopy results showed good cell adhesion and proliferation, and extensive filopodium-like protrusion connected to hydroxyapatite particles.