In shaken cultures of Phanerochoete chrysosporium, different Tweens gave rise to similar and high lignin peroxidase (UP) activities. The polyoxyethylene-sorbitan (POE-S) moieties isolated from Tweens gave rise to somewhat lower LIP activities, whereas fatty acids isolated from Tweens gave rise to much lower LiP activities than parent Tweens. LiP activity appeared 3 d after addition of Tween 80 if this was added within the first 4 d after inoculation. Of the three chemical moieties contained in Tweens, ie., fatty acids, sorbitan, and polyoxyethylene (POE), only the latter one significantly stimulated the LiP activity of the culture. The stimulatory effect of POE on the LIP activity increased till its molar mass of approx. 1 kDa, then it levelled off. The quantity of POE in the culture decreased with time. Tween 80, its POE-S moiety and POEs seem to enhance UP production and not only their release.