Recently, Uncaria tomentosa (Willd.) D.C. has become known as a healing plant with an ethnomedicinal background. There have been several reports on its constituents, in particular, oxindole alkaloids. It was found that 2 chemotypes of Uncaria tomentosa with different alkaloid patterns occur in nature. The roots of one type contain pentacyclic oxindoles and the other contains tetracyclic oxindoles. This difference should be considered when the plant is to be used for medicinal applications. Tetracyclic oxindole alkaloids act on the central nervous system, whereas pentacyclic oxindole alkaloids affect the cellular immune system. Recent studies have shown that the tetracyclic alkaloids exert antagonistic effects on the action of the pentacyclic alkaloids. Mixtures of these 2 types of drugs are therefore unsuitable for medicinal uses.