The transketolase activity (TKA) of red cells was determined in 286 controls and 50 patients with uremia. The TKA of the latter tended to exceed that of the controls, but statistically was not significant. In the uremic patients, the showed a positive correlation with serum creatinine, urea and potassium levels, and a negative correlation with the patients’ hemoglobin and TKA hematocrit. A 10-hour dialysis did not affect the TKA of the red cells in uremic patients, nor did uremic plasma inhibit or activate the TKA in vitro.