The changes in inosine 5'-monophosphate (IMP) and adenine nucleotides were studied in human erythrocytes incubated with glucose alone, or with glucose plus inosine in the presence or absence of monoiodoacetate, a strong inhibitor of the glycolytic pathways of the cells. HPLC was useful in analyzing the changes in these nucleotides. When the cells were incubated with glucose and inosine for several hours at pH 7.0, 37 degrees C, they produced much IMP, while the cells incubated with glucose and monoiodoacetate accumulated AMP rapidly within 2 hours, accompanied with the decomposition of ADP and ATP; but IMP was not accumulated. When the cells were incubated with glucose and inosine in the presence of monoiodoacetate, AMP increased more rapidly to about 1400 mu mols/ml of cells within 1 hour and then gradually decreased. ATP was consumed completely and ADP decreased correspondingly. However, IMP was produced in this case, though its accumulation was not so high as in the case with glucose plus inosine without monoiodoacetate. The production of IMP depended on the pH of the cell suspension, i.e., more IMP was produced at higher pHs.