MOLYBDENUM complexed as silico-molybdate has been shown to be very active in the re-activation of aldehyde oxidase from which the molybdenum has been removed by dialysis1. Other reports have implicated silicate, phosphate and other anions as agents which increase the activity of molybdate -dependent enzymes2,3. Jacobs and Sanadi4 suggested that a silico-molybdate complex was involved in a terminal electron transport system that was coupled with phosphorylation. It was shown by Glenn and Crane5, however, that the in vivo functioning of aldehyde oxidase was probably not dependent on silico-molybdate even though it did greatly enhance the activity of the in vitro system.