The changes in red cell organic phosphate compounds, in glucose consumption and in red cell membrane ATPase activity were investigated in A1 red cells exposed to anti A antibody. A1 red cells, incubated in anti A serum, were shown to have significantly decreased ATP content, increased content of ADP and of inorganic phosphate, as compared to red cells incubated in the same manner in autologous serum. The consumption of glucose, the content of 2,3‐diphosphoglycerate and of hexose diphosphates in antiserum‐exposed red cells were unchanged. The activity of red cell membrane ATPase in ghosts isolated from anti A serum‐exposed A1 red cells, was significantly higher than that in autologous serum‐exposed red cell ghosts. No significant differences were seen between the results in active and inactivated anti A serum. Some evidence is given and discussed that the decrease of ATP and the increase of ADP and inorganic phosphate in antiserum‐incubated red cells are due to the increased activity of red cell membrane ATPase.