Translationally controlled tumour protein (TCTP) is involved in malignant transformation and regulation of apoptosis. It has been postulated to serve as a guanine nucleotide exchange factor for the small G-protein Rheb. Rheb functions in the PI3 kinase/mTOR pathway. The study presented here was initiated to characterise the interaction between TCTP and Rheb biochemically. Since (i) no exchange activity of TCTP towards Rheb could be detected in vitro, (ii) no interaction between TCTP and Rheb could be detected by NMR spectroscopy, and (iii) no effect of TCTP depletion in cells on the direct downstream targets of Rheb could be observed in vivo, this study shows that TCTP is unlikely to be a guanine nucleotide exchange factor for Rheb.