The biosynthesis of sesquiterpenoids in Andrographis paniculata callus tissue cultures and in a derived cell-free system has been investigated. The mechanism of trans:cis isomerisation of the C-2, C-3 double bond of farnesol was shown not to involve the aldehydes as intermediates. The mode of cyclisation of farnesyl pyrophosphate in the formation of the bisabolane sesquiterpenoids, paniculides A, B and C, has been elucidated by a combination of and radioactive tracer techniques. Gyclisation of 2-cis, 6-trans-farnesyl pyrophosphate produces Z-Y-bisabolene which is oxygenated by the cultures to the paniculides. The role of the amino acid leucine in terpenoid biosynthesis has been studied. It was shown that incorporation of radioactivity from labelled leucine into terpenoids takes place via breakdown to acetyl coenzyme A and acetoacetate, and that in this case, isoprenoids are produced by the established route via mevalonate. The presence of the enzyme leucine 2,3-amino mutase has been demonstrated in the cell-free system. The B-leucine produced by this enzyme has the (3S)-configuration. The reaction is reversible and does not depend on coenzyme B12.