A method is described for the quantitative estimation of clinically important monosaccharides in plasma or whole blood by direct densitometry of chromatographically-separated zones on silica gel layers. Simple modifications of technique originally introduced to improve the reproducibility of paper chromatography have now been adapted for thin layers. The present method is based on peak height measurement with an internal marker correction. Galactose, fructose, D-xylose, and 3-O-methyl glucose can be estimated in addition to glucose, either singly or in combination, within three or four hours, using an initial sample volume of 0.5 ml. With reasonable experience and skill a coefficient of variation of 3 to 6%, depending on sugar concentration, can be achieved without replication, and the limit of sensitivity is about 0.05 mmol/l. When the performance was compared with an automated glucose oxidase/peroxidase system for glucose and the recovery for the other monosaccharides was calculated, the results were satisfactory.