Abstract Using a densimeter technique, the effect of tannic acid and n ‐butanol on the permeability to non‐electrolytes of erythrocytes of various species was studied. If both tannic acid and butanol decrease the permeability it is suggested that a carrier mechanism is involved. If tannic acid has little or no effect and butanol increases the permeability it is suggested that the penetration of such a molecule depends only on diffusion. According to this theory carriers are involved in the following systems: Urea — mouse, sheep, pig, ox and man; Ethylene glycol — man and possibly mouse; Glycerol — man, mouse, rabbit; Erythritol — mouse and possibly man; Ribose — mouse, man; Mannose, glucose, sorbose — man. No carriors are involved in the following systems: Urea — chicken; Ethylene glycol — pig, chicken, ox, sheep; Thiourea — rabbit, chicken, sheep, man, pig, ox; Glycerol — chicken, pig, sheep, ox; Erythritol — rabbit; Ribose — rabbit.