Stainless steel microspheres, with weights between 4.7 x 10/sup -9/ and 3.9 x 10/sup -8/ gram, are isolated on methyl methacrylate films. After measurement of particle diameters by optical microscopy, the specimens are encapsulated in lead foil and irradiated with an intense flux of neutrons. Radiochemical procedures are used to isolate the Mo/sup 99/, Cr/sup 51/, Fe/sup 55,59/, and Np/sup 239/ fractions. The abundance of molybdenum, chromium, and iron in the individual spheres is determined from the measured activities. The experimental error is attributed to three principal sources. Uncertainty in particle diameters as measured by optical microscope produced a relative error of 6 to 11%. Radiochemical variations led to a relative error of 5%. Contamination of the sample was successfully controlled in the Mo and Cr analyses. Impurities in the encapsulating lead foil were the largest source of error in the iron determination. (auth)