The military explosive N-methyl-N,2,4,6-tetranitroaniline (tetryl) is a direct acting mutagen in three microbial mutagenicity test systems at concentrations as low as 5 microgram/ml. In two of these systems (Neurospora crassa and Salmonella typhimurium) it caused base-pair substitution reversions. In the third (Saccharomyces cerevisiae D4) it caused mitotic gene conversions at two loci. Three nonaromatic explosives (PETN, RDX, HMX) did not exhibit mutagenic activity in the Ames Salmonella test. Further studies on tetryl seem indicated, as past and present human exposure may entail a health risk.