Ellipticine (E) and many of its derivatives are pyridocarbazole alkaloids known for their antitumour properties 1,2. 9-Hydroxyellipticine (9-OHE) is particularly active after the formation of a quaternary pyridine nitrogen (N2-methyl-9-hydroxyellipticinium acetate 3,4). Inherent difficulties in the synthesis of these molecules 5 explain the importance of obtaining them from cultured plant cells in vitro. We have previously described the bioproduction of two of them, ellipticine (E) and 9-methoxyellipticine (9-OCH3E), by Ochrosia elliptica cells 6. The aim of our continuing research is the selection of strains that accumulate large amounts of these two alkaloids and, if possible, strains accumulating 9-hydroxyellipticine. These selections are obtained by cloning and require the use of a simple assay method for rapidly screening a large number of callus samples. As the dry weight of a callus is only a few milligram and a callus usually contains very small amounts of alkaloid, the assay method must be extremely sensitive. Several derivatives of 6H-pyrido[4,3-b]carbazole have been separated and assayed by high-performance liquid chromatography (HPLC) 7,8, but the detection methods used make them ineffective for assaying alkaloids in the strains for the following reasons: the sensitivity of spectrophotometry is insufficient 7; the fluorometric detection proposed by Muzard and Le Pecq 7 requires extraction after HPLC, followed by acetylation, and in addition it is applicable only to hydroxylated derivatives; and electrochemical detection also is applicable only to the hydroxylated derivatives 8. The method proposed here involves the thin-layer chromatographic (TLC) separation of alkaloids on silica. The chromatograms are read by fluorodensitometry on a plate impregnated with dimethyl sulphoxide (DMSO), with a detection sensitivity in the range 40-300 fmol (10-15 mol) of alkaloid. This method can be used to assay alkaloids in methanol extracts of tissue cultures in vitro with no prior purification step, thereby resulting in a simple and rapid screening assay.