We report here the isolation and purification of a genotoxic antibiotic from the culture medium of a Streptomyces strain. The antibiotic was identified as actinomycin X2 by using a database search (AntiBase). In the previous studies a related compound, actinomycin D, has been shown to be non-mutagenic in the salmonella/microsome assay and several other bacterial systems. The fact that an actinomycin was detected by a bacterial repair assay seems to contradict these results. Therefore we tested several actinomycins by differential killing assay based on the same Escherichia coli strains that were used in our screening bioassays. According to our results the uvrA, recA double mutant sensitizes E. coli to the genotoxic effects of actinomycins.