An intravenously administered fluorescent carbocyanine dye, DiOC7(3), has been evaluated for use in conjunction with Hoechst 33342 as a marker of murine tumour vasculature. DiOC7(3) stains cells immediately adjacent to blood vessels and thus, like Hoechst 33342, outlines perfused tumour vasculature. The different fluorescence excitation and emission properties of DiOC7(3) and Hoechst 33342 permit discrimination of the stains in the same tissue section. Mice tolerate a DiOC7(3) dose of 1 mg kg-1 i.v. with no ill effects. The dye has a distribution half-life in blood of 180s and staining of perivascular tumour cells is sufficiently stable to allow visualisation of vasculature for up to 30 min after DiOC7(3) injection. However, DiOC7(3) causes a 75% reduction in tumour blood flow as measured by laser Doppler techniques. Consequently, the compound appears to be most suitable as a second vascular marker, administered at some time after Hoechst 33342, to detect temporal and spatial fluctuations in tumour perfusion.