That skin washing/decontamination may increase percutaneous absorption is commonly referred to as the 'wash-in' (W-I) effect. This article traces the development of the W-I effect both in vivo and more recently in vitro. The mechanism(s) responsible for this effect are examined particularly in relation to the laboratory method used in vitro. The importance of the W-I effect is presented and it is recommended that caution be practiced when skin is washed as the W-I effect may increase both local cutaneous and general systemic toxicity. Experimental data on a wide variety of chemicals are urgently needed.