Abstract
Fourteen of twenty-one naturally occurring amino acids inhibited competitively the utilization of tryptophan for growth by an indole- or tryptophan-requiring strain of Neurospora crassa. At less than inhibitory ratios of amino acid to tryptophan, certain amino acids enhanced the growth of this mutant. Growth on indole was neither enhanced nor inhibited by amino acids other than tryptophan. The uptake of tryptophan from the medium was inhibited by phenylalanine. The effects of amino acid on the growth of this strain with tryptophan can be explained by an inhibition of tryptophan can be explained by an inhibition of tryptophan uptake from the medium.