Ames testing of Direct Black 38 parallels carcinogenicity testing.

Studies have established that Direct Black 38 and two other benzidine-based dyes are carcinogenic. The carcinogenic effect has been generally considered attributable to the metabolic release of benzidine from Direct Black 38 and similar dyes. However, Ames tests indicated that when Direct Black 38 is reduced with sodium dithionate it is more mutagenic than can be accounted for by complete release of all the benzidine present in the dye molecule. While most dyes are not mutagenic when tested with S-9, a series of benzidine congener dyes were all found to be mutagenic with either TA 98 or TA 100 strains, if the dyes were first reduced with sodium dithionate. Unreduced dyes were not mutagenic. Neither anaerobic conditions nor addition of riboflavin induced mutagenicity of these dyes under the condition of our experiments.