"Malonamate shuttle" as a model of nitrogen flow by malonamate in Bradyrhizobium japonicum-soybean symbiosis has been proposed. One of the key enzymes of the shuttle system, malonamidase E1a, was purified in an electrophoretically homogeneous form as a constitutive protein from free living Bradyrhizobium japonicum USDA110. The molecular size of the enzyme was 129,000 dalton composed of two identical 59,000 dalton subunits. pI of the enzyme was 5.4. The enzyme was highly specific for malonamate and malonate as substrates and no cofactors or external ions were required for its catalysis. $K_m$ for malonamate and for malonate were 4.2 and 12.5 mM, respectively. Optimal pH for malonohydroxamate formation from malonamate and $NH_2OH$ and for that from malonate and $NH_2OH$ were about 8.0 and 6.5, respectively.