It has been shown that the spontaneous revertants in the Ames test can depend on the amount and type of bottom agar on the plates. The most clear-cut effect was found with his G46 strains, carrying the ??uvrB mutation, and with the new hisG428 strain, TA102. Evidence of mutagenic impurities in agar has been found. Chemicals/CAS: agar, 9002-18-0; histidine, 645-35-2, 7006-35-1, 71-00-1; Culture Media; Histidine, 71-00-1; Mutagens.