Inherent complexity and instability of enzyme preparations make deceptively‐simple enzymic methods more vulnerable to error than methods using pure stable chemicals. Hence, four D‐amino acid oxidase (DAAO) preparations from three commercial sources were compared for enzymatic effectiveness. Per unit of claimed activity, the four DAAO enzyme preparations differed by <2 to >100 fold in apparent ability to oxidize “reactive”;14C‐D‐methionine and “unreactive”;14C‐D‐lysine in pure solution. The data illustrate how use of such differing preparations on less‐pure soil or plant extracts, which may contain unknown amounts of enzyme‐inhibiting or ‐inactivating components, would produce confounding results and permit erroneous conclusions. Implications are that users of commercially available DAAO preparations must acquire large enough lots of homogenous enzyme to establish effectiveness on the most resistant substrates of interest and still have enough for all experimental assays where direct relative comparisons are required.