Summary: A highly selective and specific method for determining caffeine levels in plasma and saliva is described. The method is a microscale procedure that requires only 25 mUl of specimen, although the assay can be performed with as little as 10 mUl of the specimen and is therefore, particularly suitable for monitoring caffeine levels in the neonates. The method is sensitive to 0.5 mUg/ml and is specific for caffeine as none of the other xanthines shows any interference. The method is also fast as treatment of specimen requires only precipitation of proteins and thus is ideal for use in hospital and clinical laboratories.