The aim of this work was to compare the molecular properties of pyrophosphate:fructose 6-phosphate 1-phosphotransferase (PFP) and ATP:fructose 6-phosphate 1-phosphotransferase (PFK). Both enzymes were purified to apparent homogeneity from potato tubers (Solanum tuberosum cv Record). Neither PFP nor PFK preparations contained detectable activity of the other enzyme. PFP was composed of two polypeptides of apparent molecular weight 58,000 and 55,700 whereas PFK contained four polypeptides of apparent molecular weight between 46,300 and 53,300. Chemical cleavage of individual PFP and PFK polypeptides gave a different set of fragments for each polypeptide. On Western blots antisera against PFP failed to cross-react with any of the four PFK polypeptides, and antibodies against PFK failed to bind to either of the PFP polypeptides. Antibodies that immunoprecipitate PFP activity had no effect on PFK activity. Conversely, antibodies against the four PFK polypeptides precipitated the activity of PFK, but not that of PFP. This work shows that potato tuber PFP and PFK are composed of distinct, unrelated polypeptides and indicate that interconversion between PFP and PFK is unlikely.