The potential use of the protein denaturant guanidine hydrochloride to inhibit selectively the enzyme activity of serum alkaline phosphatase isoenzymes from liver, bone, intestine, and placenta was investigated. Inhibition of each isoenzyme was shown to be dependent on time and concentration of inhibitor. In the presence of 0.3 mol/l (28.7 g/l) guanidine hydrochloride for 170 seconds 14%, 47%, and 90% of the total alkaline phosphatase activity remained in samples of bone, liver, and intestinal origins, respectively. In contrast, the activity of the placental isoenzyme increased by 24%. The degree of inhibition was shown to be independent of total alkaline phosphatase activity. Investigations were performed at 37°C using the Cobas Bio centrifugal analyser. We conclude that this reagent has several practical advantages over urea as a selective inhibitor of alkaline phosphatase isoenzymes, including a faster and more reproducible inhibition at a much lower reagent concentration.