Cancers contain a 'side population' (SP), a subset of cells that is greatly enriched in stem cells and which contains malignant progenitors. SP cells are characterised by high efflux capability for Hoechst 33342 dye and for anti-cancer therapeutic agents through transporters; ABCG2 (ATP-binding cassette transporter G2) is currently most closely associated with the SP phenotype. Guanosine is an important intercellular signalling molecule; it stimulates stem cell proliferation in vivo and affects cholesterol efflux in vitro through activation of ABCG transporter (ABCG1), raising the possibility that it might also affect ABCG2 and hence the SP. We examined the effects of guanosine on the SP of A549 lung cancer cells. Fluorescence-activated cell sorting (FACS) revealed that exposure to 10 microM guanosine significantly decreased the proportion of SP cells after 48 hours but not after 6 hours. In contrast, Western blot analysis showed that 10 microM guanosine significantly decreased ABCG2 expression after 6 hours, but not after 48 hours. These data demonstrate that guanosine affects both the proportion of SP cells and ABCG2 transporters, but the lack of correlation between ABCG2 expression and the SP phenotype indicates that transporters other than ABCG2 are involved in maintaining the SP phenotype in A549 lung cancer cells.