In this report, the results of our laboratory and field experiments are summarized and discussed to show the influence of soil microbial activities on the bioavailability and biotransport of transuranics in desert environments. DISCUSSION We reported previously that Aspergillus niger, a commonly occurring soil fungus, took up plutonium from culture media and transported it to the fungal spores (Au, 1974). The chemical form of the plutonium added to the culture media, and the pH of the growth media, had a marked influence on plutonium uptake and translocation (Au and Beckert, 1975a). To better perceive the concept of plutonium transport, we used the term transport factor (TF) which is applicable for culture media where the distribution of nutrients and pollutants is uniform (Au et al., 1976a). The concentration-independent transport factor is defined as the fraction of the total plutonium that is transported from the medium to the tissue divided by the fraction of the total dry mass transported from the medium to the tissue, or TF where, - Pu, /Puy ML/My Pup total plutonium content of tissue (e.g., mycelium, spores) Puy total plutonium originally present in the parent medium. My M dry mass of tissue dry mass originally present in the parent medium. The TF shows immediately if accumulation of or discrimination against the pollutant has occurred: TF > 1 indicates accumulation, and TF < 1 defines discrimination against the pollutant. In general, it was found that for Aspergillus niger all transport factors defining the movement of plutonium from culture media via the mycelium to the spores were smaller than 1, indicating discrimination against the transport of plutonium from the agar media to the spores. This was true for all chemical forms of plutonium tested at pH 2.5 and 5.5. As required by the definition, the transport factors derived from experiments using plutonium dioxide spheres were fairly concentration-independent, although the plutonium concentrations applied to the culture media varied by a factor of 100. The TF values derived from experiments using plutonium nitrate and plutonium citrate, respectively, were reasonably close at pH 2.5, but showed a greater variation at pH 5.5 (Beckert and Au, 1975). 220