SUMMARY: Bacillus subtilis, grown on nutrient broth, casein hydrolysate, or on a mixture of amino acids simulating casein hydrolysate, yielded washed suspensions with an active reductive amination system. On simple defined media the activity of the organisms was relatively low. Additions of a variety of amino acids, singly or in simple mixtures, yielded suspensions with activities up to about half of that attained with complex media. The performance of 18 amino acids in about 200 different combinations was analysed statistically; no amino acid showed marked and significant stimulation of enzyme formation. It was concluded that a high enzyme yield requires a medium with a complex amino acid relationship, and that the performance of complex media cannot be explained directly in terms of a few active components.