Solute transfer across the cellophane membrane of the artificial kidney is known to occur by dialysis and by ultrafiltration. During the course of our studies on iron transport, we presented data to this Society last year, indicating that radioiron in tap water is bound to cellophane and preferentially removed by a carrier in blood, presumably transferrin (1). A third mechanism of solute transport across cellophane membranes was thus identified. As a further extension of this study, we have evaluated 17 metals to determine their affinity for cellophane and whole blood.