A modern aqueous-ethanolic formulation of the roots of Pelargonium sidoides (Eps 7630), elaborated from the traditional herbal medicine used in areas of southern Africa, is effectively employed for the treatment of ENT and respiratory tract infections in modern phytotherapy. Previous studies have demonstrated antibacterial and immunomodulatory activities. To gain insight into the mode of action at the molecular level, gene expression analyses for the inducible nitric oxide synthase and the cytokines interleukin (IL)-1, IL-12, IL-18, tumour necrosis factor (TNF)-alpha, interferon (IFN)-alpha, and IFN-gamma, were performed using reverse transcription-polymerase chain reaction (RT-PCR). The experiments were carried out in parallel in non-infected and in Leishmania major-infected RAW 264.7 cells and the expression profiles were compared with those mediated by IFN-gamma+LPS. Eps 7630 induced low mRNA levels in non-infected cells, and it considerably up-regulated the transcript expressions in parasitised cells. Interestingly, and in contrast to activation by IFN-gamma+LPS, Eps 7630 also stimulated infected cells to produce IFN-gamma mRNA. A similar expression profile was observed for the methanol-insoluble fraction (MIF) of Eps 7630 and gallic acid, a trace constituent of the extract, while the methanol-soluble fraction and umckalin, an exclusive and representative member of the occurring coumarins, proved to be devoid of any remarkable gene-inducing capabilities. The present results provide not only convincing support for the improvement of immune functions as previously demonstrated in functional bioassays, but also evidence for activation at the transcriptional level and suggest that the underlying inducing principle is located in the MIF.