One of the non-exacting purineless mutants derived from Bacillus megaterium IAM-1245 by X-ray irradiation accumulated 5-amino-4-imidazole carboxamide riboside (AICA-riboside). On the stock culture of this strain, especially on the polypeptone agar slant, back mutants were found so many that they degrade the yield of AICA-riboside. The effects of many medium constituents on the growth and productivity of AICA-riboside of this strain were studied for establishing optimum commercial medium. Among cations, potassium-, ferrous-, manganese-, magnesium-, copper-salt increased the accumulation of AICA-riboside at each suitable concentration respectively. L-Histidine promoted the growth remarkably and made the yield lower. L-Isoleucine, L-valine and especially L-threonine elevated the yield. When they were mixed together highest yield was obtained. So it will be convenient to use Aji-Eki (soybean acidhydrolysate, containing several amino acids) for commercial medium. As for purine source, adenine, inosine and RNA were preferable becauce if purines used had any substituents on their 2nd carbon atom of guanine or xanthine the accumulation of AICA-riboside was markedly decreased.
