ras oncoproteins and ras-related proteins constitute a large family of the small GTP-binding protein family. The rab branch of the ras superfamily is involved in the intracellular transport along the secretory and endocytic pathway in eukaryotic cells. We here demonstrate that a member of the rab branch, the rab2 protein, is frequently overexpressed in peripheral blood mononuclear cells from patients with solid neoplasms. Moreover, this expression is shown to be greatly modified during the course of therapy. Our results provide strong evidence for the implication of a small GTP-binding protein in immunological events associated with neoplastic diseases. The precise cellular population involved as well as the potential prognostic value of this process remains to be determined.