MICRO-ORGANISMS prevent over-production of amino -acids by two mechanisms: (a) end-product inhibition of the action of the first specific enzyme in the biosynthetic sequence, and (b) inhibition of the synthesis of all the enzymes in the sequence (enzyme repression)1,2. A number of growth-inhibitory analogues of normal protein amino-acids, besides being incorporated into proteins, are capable of mimicking the corresponding amino-acid in operating these biosynthetic control mechanisms (see reviews by Cohen3 and by Fowden, Lewis and Tristram4).