Abstract 1. 1. The pig enzyme was purified to homogeneity and was found to be a tetramer of apparently identical subunits. 2. 2. The pig enzyme was found to contain 1 mol NADH/mol enzyme which is tightly bound, which is not directly involved in catalysis and which so far has not been removed from the enzyme so as to produce an active apoenzyme. 3. 3. The pig enzyme seems to contain only one functioning active site/tetramer. 4. 4. The pig and sheep enzymes are compared in respect to NADH binding, substrate specificity, immunological response and surface charge.