Alternate adsorption of manganese oxide nanoparticles with polycations poly(dimethyldiallyl-ammonium) (PDDA) or myoglobin (Mb) onto silver, quartz, and rough pyrolytic graphite gave stable, porous, ultrathin films. Quartz crystal microbalance (QCM) and UV- vis absorbance revealed regular film growth at each adsorption step for MnO2 and PDDA and for SiO2 nanoparticles and Mb. Scanning electron microscopy of MnO2/PDDA films showed smooth surfaces on the 20 nm scale and cross sections consistent with individual nanoparticles. QCM during growth of films of Mb and MnO2 reflected a competition for adsorption of the protein by the film surface and dispersed MnO2 nanoparticles. Nevertheless, films of Mb and MnO2 up to 30 nm thick on rough pyrolytic graphite electrodes could be constructed These novel films featured reversible interconversion of the protein's heme Fe-III/Fe-II redox couple with 10 electroactive layers of protein, considerably more than for polyion-Mb films on smooth gold (ca. 1.3 electroactive layers), and coiled PSS/Mb, films on rough graphite (7 electroactive layers). Shifts in redox potential caused by CO complexation of the heme Fen, BET specific areas, and electrochemically driven catalytic reduction of oxygen suggest that the Mb/MnO2 films are highly porous to gas molecules. To our knowledge, these films represent the first nanofabrication of inorganic particles with functional proteins by the layer-by-layer method.