—Pig brain glutaminase (EC L-glutamine amidohydrolase) has been purified about 5000-fold from acetone powder. Glutaminase exists in different molecular forms, dependent on the ionic composition of the buffer. The three main forms are similar to those of kidney glutaminase and therefore called the tris-HCl enzyme, the phosphate enzyme, and the phosphate-borate enzyme. The sedimentation coefficients, as estimated by sucrose gradient technique, are 7·3, 8·7, and 53, respectively. Glutaminase has a pH optimum of about 9, but the pH curves of the tris-HCl enzyme and the phosphate-borate enzyme have different shapes. The apparent pK1 of the tris-HCl enzyme-substrate complex is similar to pK2 of inorganic phosphate, the apparent pK2 of both the tris-HCl and the phosphateborate enzyme complexes is similar to pK2 of glutamine. By use of the electron microscope we were able to see the phosphate-borate enzyme.