Solarium aculeatissimum Jacq. contains the steroid saponins aculeatiside A and aculeatiside B at high levels in the root, whereas they are lacking in all organs of tomatoes (Lycopersicon esculentum Mill). We examined the site of synthesis of these steroid saponins by using grafts between S. aculeatissimum and tomato plants. When S. aculeatissimum was grafted onto a stock of tomato (S.a./Tom.), a small amount of steroid saponin was found in the leaves and the stem of S. aculeatissimum but not in the roots of the tomato. However, when tomato was grafted onto stocks of S. aculeatissimum (Tom. /S.a.), steroid saponin occurred only in the roots of S. aculeatissimum. These results suggest that the steroid saponins are mainly synthesized in the roots of S. aculeatissimum. The lack of transport of steroid saponins from the roots of S. aculeatissirnum to the leaves and stem of the tomato remains unexplained.