Extracts of liver and muscle convert citric acid into laevo-rotatory isocitric acid [Martius, 1938]. As the rotation of isocitric acid, like that ofother ac-hydroxy acids, is much increased by (NH4)2MoO4*, the polarimeter can be used to measure low concentrations of isocitric acid in tissue extracts [Martius, 1938; Jacobsohn, 1940]. On applying this method to plant extracts we met with an unexp...