Polyhydroxyalkanoates (PHA), which are produced by many microorganisms, are promising polymers for biomedical applications due to their biodegradability and biocompatibility. In this study, we evaluated the suitability of medium chain length (mcl) PHA as surface materials for immobilizing proteins. Self-stabilized, artificial mcl-PHA beads with a size of 200-300 nm were fabricated. Five of six tested proteins adsorbed nonspecifically to mcl-PHA beads in amounts of 0.4-1.8 mg m(-2) bead surface area. The binding capacity was comparable to similar-sized polystyrene particles commonly used for antibody immobilization in clinical diagnostics. A targeted immobilization of fusion proteins was achieved by using inactive extracellular PHA depolymerase (ePHA(mcl)) from Pseudomonas fluorescens as the capture ligand. The N-terminal part of ePhaZ(MCL) preceding the catalytic domain was identified to comprise the substrate binding domain and was sufficient for mediating the binding of fusion proteins to mcl-PHA. We suggest mcl-PHA to be prime candidates for both nonspecific and targeted immobilization of proteins in applications Such as drug delivery, protein microarrays, and protein purification.