Monofluorophosphate (MFP) can be synthesized in vitro by the transfer of a phosphoryl group from ATP to F-, catalyzed by pyruvate kinase, or by the autocatalytic transfer of a phosphoryl group from the enzyme phosphoglucomutase. There is, however, no evidence that MFP is a normal intermediate in cell metabolism. MFP can be degraded by both alkaline and acid phosphatases, and the mechanism is probably similar to the hydrolysis or transfer of phosphoryl groups from other phosphatase substrates. MFP competitively inhibits pyruvate kinase and alkaline phosphatase, and irreversibly inhibits phosphorylase phosphatase. The possibility that impurities or hydrolysis products are responsible for reported effects makes the interpretation of many inhibition studies difficult. The widespread therapeutic use of MFP presents a strong case for a more detailed study of its metabolism.