It has previously been shown that pig liver monoamine oxidase (monoamine: O 2 oxidoreductase, EC 1.4.3.4) can be rendered soluble in buffer by extraction with methylethyl ketone. The soluble enzyme can be rebound to delipidated membranes in the presence of highly acidic phospholipids. A comparison of the thermostabilities of the enzyme bound to the mitochondria and the soluble form showed that the soluble enzyme was much more labile than the bound enzyme. Rebinding of the soluble enzyme to relipidated membranes partially restored the thermostability. The soluble enzyme was more sensitive to tryptic digestion than the enzyme bound to mitochondria. Rebinding of the soluble enzyme to replipidated membranes did not decrease its sensitivity to trypsin. The role of membrane components in stabilizing the enzyme molecule is discussed. The change in the rate of tyramine oxidation with temperature was the same for both forms of the enzyme. Harmaline was found to reversibly inhibit both bound and soluble enzyme to the same degree. This was true both with tyramine and serotonin as substrate. However, serotonin oxidation by either form of the enzyme was inhibited more than tyramine oxidation. Pargyline irreversibly inhibited tyramine and serotonin oxidation to the same degree, both with the bound and the soluble enzyme. The molecular activity of the enzyme did not change after liberation from the membrane. The possibility that there are multiple forms of monoamine oxidase in the pig liver is discussed in the light of these findings.