Several preparations of Cremophor E1, several of other non-ionic detergents and several components of Cremophor E1 were tested for their histamine-releasing capacity in dogs. Lutensol AP 10 and a derivative of 1,2-propylenglycol were ineffective, but showed excellent properties as detergents. Thus the histamine-releasing capacity was not necessarily combined with the tenside effect of the surfactants. Oleic acid found in Tween 80 as well as in Cremophor E1 seems to be the most effective constituent, but the alcohol seems also to be important for the histamine-releasing capacity. The development of a non-toxic solubilizer for lipophilic drugs seems of considerable clinical interest.