Fifteen strains of bacteria were treated with ultraviolet light or N-methyl-N'-nitro-N-nitrosoguanidine to derive auxotrophic mutants, which were screened for their ability to produce L-threonine. A number of auxotrophs were derived from each strain. Among them, those which produced a large amount of L-threonine were found in Aerobacter aerogenes, Serratia marcescens and Escherichia coli, the members of the family Enterobacteriaceae. Nutritional requirements of these threonine producers were proved to be methionine, lysine, or α, ε-diaminopimelic acid (DAP).
In A. aerogenes and E. coli, double and triple auxotrophs were derived with futher mutational treatment. As α rule, imposition of additional block led to the increase of L-threonine production. In E. coli, many triple auxotrophs (DAP-, Met-, De-) and their isoleucine revertants were screened for their ability to produce L-threonine. Enhancement of L-threonine production was achieved with these mutants.
One of the isoleucine revertants, KY8280, was used to investigate some cultural con-ditions. As a result, L-threonine accumulation reached to a level of 13.8 m, g/ml with the medium containing 7.5% fructose.

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