A biologically active composition made up of core particles having diameters of less than about 1000 nanometers which are coated with a layer which is designed to allow attachment of biologically active proteins, peptides or pharmacological agents to the microparticles. When viral protein is attached to the core particles, the result is a viral decoy which accurately mimics the native virus in both size and structure while being entirely devoid of virulent activity due to the microparticle core. Other antigenic proteins or peptides are attached to provide molecules which are useful in raising antibodies or as a diagnostic tool. Further, pharmacological agents are attached to the microparticles to provide pharmaceutical compositions.