was the The major problem encountered in these types of quantitative studies contaminant prevention of cross-contamination of collected material with the contained in the culture medium and in the parent mycelium. Cross-contamination ed the was eliminated by the use of a sample collection technique that permitt other small selective massing of aerial fungal spores, as well as a variety of collection simple and new This 1975b). , particles, on a filter (Au and Beckert technique was extensively evaluated at this Laboratory using Aspergillus niger grown on a buffered agar medium and on soil, both of which contained transuranic elements. Microscopic examinations of the filter contents showed the absence of mycelial and conidiophore (stalk) fragments. Because this technique prevented any direct contact between the spores and the medium containing the transuranic element, reliable and accurate transfer coefficients from medium to spores could be established for the first time. The soil fungus Aspergillus was chosen as our test organism because it is ubiquitous and is morphologically well described (Raper and Fennell, 1965). More importantly for our experiments, it produces aerial spores atop lengthy stalks. This feature prompted the development of the collection technique mentioned above. Aspergillus is also present in the Nevada Test Site (NTS) soils; however, it is certainly not the only important soil microorganism in these soils. In fact, the soil fungal and bacterial populations of desert soils can be surprisingly high and diversified even in locations without plants. As plant growth increases, the number of the soil microorganisms increases, and their relative abundance may vary. Soil microbial surveys conducted in Area 13 of the NTS showed that in the 0-5-cm soil segment the fungal and bacterial populations were generally 2 to 4 times as high in the hummocks as in the soils without plant growth (Table 1). When the desert soil was cultivated and planted to produce crops under greenhouse coverings to simulate more typical agricultural conditions, the total fungal and bacterial numbers in the 0-6-cm soil segment increased during the growing season by factors of 3 and 16, respectively (Au et aZ., 1976b). The averaged results are listed in Table 1, together with the relative abundance (in percent) of the major fungal genera as determined in these soils. Also included in Table 1 are the fungal and bacterial biomasses expressed in kilogram per hectare (kg/ha) and calculated very conservatively according to the method used by Alexander (1961). The biomass figure might provide a better appreciation of the potential of the microbial activities in these soils. How can soil microorganisms influence the transfer of transuranics from soil to man? It has been well established that soil microbes can attack various minerals which are normally biologically unavailable to plants and can change them by a number of processes into forms available to plants (Waksman, 1927; Bollen, 1959; Alexander, 1961). A major role in these solubilization processes seems to be attributable to complexing cell exudates such as certain organic acids. It is known that many fungi, notably the black-spored Aspergtllt (Aspergillus niger, A. carbonarius, A. japonicus, and A. phoenicis), and certain species of Pentcilliuwm produce and exude relatively large amounts of nonvolatile organic acids, such as citric, oxalic, and gallic, into the culture media (Hawker, 1950; Raper and Fennell, 1965; Chmiel, 1975). Smaller amounts are produced by some species of the Mucoraceae. In addition, Aspergillus spp. 221