A procedure was developed for the isolation from soils of fractions rich in "unknown" soil-nitrogen which constitutes close to one half of the total soil-nitrogen. Three fractions rich in "unknown" soil-nitrogen were prepared and the biodegradation by soil microbes of each of these fractions in the presence of clay soil, sandy soil and pure sand was tested. It was found that the fractions required purification to remove excess salts before their biodegradability could be properly assessed. The most purified fraction, which contained a substantial portion of the "unknown" soil-nitrogen, was readily degraded by soil microbes at pH 7.0: the order of biodegradation in the three media was pure sand > sandy soil > clay soil. Our data suggest that most of the "unknown" soil-nitrogen is biodegradable. Therefore, further chemical and microbiological work on this nitrogen fraction is warranted in order to find ways to increase its utilization in soils by plant roots and microbes.