The activity of acid phosphatase(s) in rabbit erythrocyte membranes has been studied. The enzyme (s) utilize a variety of monophosphates including nucleotide monophosphates. It also exhibits phosphotransferase activity. No requirement for magnesium ions was demonstrated, and its activity is unaffected by the addition of sodium, potassium, and ouabain. In contrast to soluble phosphatase(s) of the rabbit erythrocyte it is markedly inhibited by fluoride ions. Experiments with intact cells suggest that the enzyme is oriented to the outer surface of the cell membrane. © 1969.