Abstract Studies of the effect of triton X‐100 on the fragility of rabbit erythrocytes and on shape and volume changes of rabbit and sheep erythrocytes suspended in an isotonic salt solution suggest that this subtance alters the erythrocytes of other species in a manner similar to that reported previously for human red cells. Using a densimeter technique it was found that the permeability of relatively slowly moving non‐electrolyte‐carrier‐complexes is decreased by triton X‐100 but more rapidly penetrating systems are not affected by this substance. The permeability of some molecules which cross the membrane by diffusion is increased by triton X‐100. It is suggested that triton X‐100 affects both the aqueous channels and the portion of the membrane across which carriers move. Its action is less than that of tannic acid or butanol.