The Ames mutagenicity assay with Salmonella typhimurium strains (TA97a, TA98, TA100 and TA102) was used to investigate the ability of five dyes (thiadiazole-1,3,4 derivatives) to induce point mutations. The results did not reveal any mutagenic effects of the tested dyes on the S. typhimurium strains. No mutagenic activity was observed in the experimental series with and without exogenous activation.