There is growing evidence that the pineal gland has antineoplastic properties which, however, can only partially be attributed to its hormone melatonin. While the in vivo tumor-inhibiting activity of melatonin is established, observations on its in vitro effects have been contradictory. The effect of this substance was investigated on six human cancer cell lines and compared to the activity of a partially purified, melatonin-free low molecular weight pineal extract (UMO5R). Melatonin showed hardly any effect but UMO5R was capable of inhibiting the growth of all the six cell lines tested. It is therefore concluded that a direct inhibiting action on tumor cells is not a general physiological role of melatonin as opposed to UMO5R. It will be worthwhile to purify the yet unidentified pineal antitumor activity since it may have a considerable therapeutic potential.