I analysed spent culture medium from plant cell suspension cultures for the presence of naturally occurring oligosaccharides. A potential oligosaccharin was purified from the spent culture medium of rose (Rosa sp.) cells. The oligosaccharide was purified by anion exchange chromatography, gel permeation chromatography and HPLC. Its structure was determined by paper chromatography, HPLC and NMR and found to be a trisaccharide: α-D-mannopyranosyl-(1→4)-α-D-glucuronopyranosyl-(1→2)-myo-inositol (Man-GlcA-Ins). It accumulated in the culture medium at a steady rate between 6 and 19 days after sub-culturing to reach a concentration of 0.8 μg ml^-1. Use of the radiolabelled Man-[¹⁴C]GlcA-Ins revealed no turnover of the oligosaccharide by the cells. Man-GlcA-Ins was found to be able to inhibit the incorporation of L-[U-¹⁴C]leucine into acid precipitable proteins in rose cells when presented to the cells at a concentration of 10^-5 -1.0 μg ml^-1, but was without effect on the uptake of the amino acid. It was also able to induce the formation of an acid resistant product of L-[U-¹⁴C]phenylalanine metabolism in rose cells when presented at a concentration of 10^-6 -1.0 μg ml^-1. It was found to be without effect on the gibberellin induced elongation of pea stems and on the proteins produced by the rose cells in its presence. Following the patterns of accumulation of Man-[¹⁴C]GlcA-Ins relative to that of the ¹⁴C-polymers in the spent culture medium after presentation of D-[6-¹⁴C]glucuronic acid suggested that Man-GlcA-Ins was formed by breakdown of an existing precursor molecule rather than by de novo synthesis. A possible source of Man-GlcA-Ins is a group of lipids termed the phytoglycolipids.