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SUMMARY: The morphological mutant rod-4 derived from Bacillus subtilis 168 trp can be changed from a round form to a rod by the addition to the growth medium of sufficient acid-hydrolysate of casein. The hydrolysate can be replaced by a mixture of amino acids, and the only individual amino acids giving similar results are l-glutamic acid, l-proline, l-arginine and l-ornithine. Since the lag in the action of l-glutamate was less than for the other amino acids, this amino acid is likely to be responsible for the effect of the mixture. Experiments with the l-glutamine analogue, γ- l-glutamylhydrazide, strongly suggest that l-glutamine is the active metabolite rather than the amino acid itself. The correcting effect of high ionic strengths of the growth medium on the morphology of this mutant seems to be mostly due to the increased effectiveness of l-glutamate or l-glutamine in the presence of high concentrations of salts.