The branched-chain amino acids L-leucine, L-valine and L-isoleucine can, when supplied to the medium, inhibit the growth of the duckweed Spirodela polyrhiza. The responses to Lleucine and to L-valine could be described as all-or-none responses, that to L-isoleucine as a graded response. This information permitted unambiguous measurement of the growth responses. The relation between amino acid dose (initial concentration in the medium) and growth response was evaluated by means of probit analysis. Dose response curves were determined for each branched-chain amino acid when supplied singly to the medium. They were also determined when a second amino acid was added to the medium at a fixed initial concentration. It appeared that L-glutamic acid, glycine and L-alanine, in increasing order (1) antagonized, each to a distinct degree, the growth inhibitory action of irrespective which branched-chain amino acid and (2) inhibited the uptake rate of the branched-chain amino acids. D-valine, L-ornithine and L-lysine did not antagonize, nor did they inhibit uptake. Evidence is provided that the observed antagonisms result from inhibition of the uptake of the growth inhibitory amino acids.