The precision of replicate analyses for alkaline phosphataseactivity measured on a survey reference sample was extremely poor. The reference sample's enzyme itself became suspect and was demonstrated to be sensitive to alkaline denaturation-in sharp contrast to the stability of the alkaline phosphatasesfound in human serum. The stability and chemical reactivity of the phosphatases present in this reference sample and in pooled frozen human serum, as well as those found in 4 partially purified nonhuman preparations and 4 commercial serum control materials, were investigated with regard to heat and alkaline denaturation, electrophoretic migration, and inhibition by phosphate, EDTA, and L-pheflylalanine. It was concluded that criteria of stability and chemical reactivity, as well as more detailed information concerning the source of enzymes utilized in reference samples and control materials, are needed. On the basis of these studies, reliance on commercial serum enzyme control materials as an enzyme "standard" cannot be endorsed.