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overpressure layer chromatography (oplc) is further developed for the online preparative isolation of natural compounds. Its use is demonstrated for the rapid isolation of one or two compounds in less than 1 h as well as for the isolation of more substances from plant extracts andor prepurified fractions which require a few hours. Depending on the separation problem, sample sizes between 50 mg and 0.5 g may be applied to 20x20 cm silica plates. Principles of the method, practical considerations, and advantages of this techique over other available methodologies are discussed.For many years, the most commonly used method in preparative planar chromatography has been traditional layer chromatography, requiring removal and subsequent elution of substance zones (1). Sequential thin layer chromatography (stlc) with locally and temporally variable solvent application (2,3) has also been useful for preparative purposes (4), even though it is an off-line method. One major advance in preparative planar chromatography was the development of centrifugally accelerated layer chromatography (clc), allowing on-line separations to be made. It has been shown, however, to be rather inefficient when used to solve complex separation problems ( 5 ) .Overpressure layer chromatography (oplc) is a forced-flow technique developed by Tyihak, et af. (6)(7)(8) in which the vapor phase is eliminated by completely covering the sorbent layer with an elastic membrane under external pressure. The separation is thereby carried out in a closed system with parameters that may be controlled. To date, there is information available on preparative oplc separations only in the off-line mode We now report the use of oplc on a 20-and 40-cm distance for preparative on-line (9-11). separation of naturally occurring compounds. METHODSThe chromatographic plate becomes a planar column by sealing the plate all around and by draining off the solvent. The eluent passes a detector system and is fractionated by an automatic collector. The flow scheme is depicted on Figure 1. MOBILE PHASE IWLEl I