[en] A technique is described for the production of a few micrograms of a niobium compound which is soluble in xylene and suitable for incorporation into polystyrene microspheres. Details are given of the successful routine production of the spheres in the size range 4.8 - 5.2 μm diameter labelled with ⁹²Nb, using a spinning top aerosol generator. The labelled particles are intended for inhalation by volunteers who will be used to calibrate X-ray detectors for the assessment of plutonium-239 in the human lung. Leaching experiments were therefore conducted on ⁹⁵Nb-labelled particles using a range of solvents which included hydrofluoric acid, hydrochloric acid, oxalic acid, normal saline and ethanol. Results demonstrated that there was negligible loss of the radioactive label over periods up to 30 days, suggesting that these particles once inhaled would not give rise to any appreciable systemic uptake of sup(92m)Nb. (author)