Glutaminase-asparaginase from Pseudomonas 7A appears to have four subunits with a molecular weight of 36,000 +/- 500 by sedimentation equilibrium in 5.9 M guanidine HCl and 34,000 by amino acid analysis. Analytic sedimentation equilibrium of the native enzyme showed a molecular weight of 140,000 +/- 3,300 with no signs of association or dissociation. Moving boundary and zone sedimentation in buffer showed normal behavior with sedimentation coefficients of 7.92 and 7.75 S, respectively. In contrast, the enzyme appeared to polymerize during zone sedimentation when the initial protein concentration was greater than 1 mg/ml and the buffer contained asparagine, glutamine, or 5-diazo-4-oxonorvaline. An extension of our method for active enzyme sedimentation is described which utilizes the changes in absorption during hydrolysis of asparagine by low concentrations of enzyme. Polymerization was not seen under these conditions.