Strong inhibition of the growth of S. polyrhiza occurs when L-valine, L-isoleucine or L-leucine are added singly to the culture medium in a concentration of about 5 x 10~5 M or higher. Excellent growth takes place, however, when L-valine and L-isoleucine are supplied together, also in combination with L-Ieucine. When L-leucine and L-isoleucine are added simultaneously, growth was completely inhibited, just as with the combination L-valine and L-leucine. The results are explained in terms of end-product inhibition of a common enzymatic step in the synthesis of these branched chain amino acids.