A method for isolating and characterizing individual particles containing fissionable materials is described. Membrane filters, used to collect the particles from atmospheric effluents from a nuclear processing plant, are cast into films composed of a polycarbonate matrix containing the particles. Collected particles containing plutonium or other fissionable materials are identified by fission fragment tracks produced by irradiating the film with thermal neutrons. The nature of the fissionable material is identified by coating the film with nuclear track emulsion and measuring the ratio of alpha particle tracks in the emulsion to the fission fragment tracks in the polycarbonate film. Single particles are isolated by excising small squares from the film and are prepared for sizing and electron microbeam analysis by peeling the emulsion off the film square, placing the square on a beryllium sample-mounting block, and washing the polycarbonate away from the particle.