$eta $-1, 4-D-arabinogalactanase isolated from alkalophilic Bacillus sp. HJ-12, approximate Mw 42 kDa, was generally stable in the range of pH 6-10 and below 50$circ$C and its highest activity was observed at 60$circ$C with pH 7-9. The isolated $eta $-1, 4-D-arabinogalactanase specifically hydrolyzed $eta $-1, 4-galactosyl linkage that is the major structure of soybean arabinogalactan (SAG) but not $eta $-1, 3-galactosyl linkage of the other polysaccharides. K. was estimated as 0.67 mg/ml by the method of Hanes-Woolf plot. No metals and chemical reagents inhibited the enzyme activity but urea did. The active site of this enzyme assumed to be tryptophan residue. The hydrolysis products from SAG, assayed by gel chromatography, TLC and HPLC, were predominantly galactotetraose (Gal$_{4}$) and triose (Gal$_{3}$) with a small portion. $eta $-1, 4-D-arabinogalactanase hydrolyzed ONPG as well as SAG, and the degree of hydrolysis of SAG was 15% which is lower than that by the other $eta $-1, 4-galactanases from different sources. SAG treated with this enzyme resulted in the reduction of specific viscosity up to 70%.