Emissions generated by firing the M16 rifle with the propellant WC844 in a combustion chamber designed to simulate conditions of actual use were tested for mutagenic activity in the Salmonella/Ames assay. Dimethyl sulfoxide extracts of emissions collected from either the breech or muzzle end of the rifle were mutagenic in three strains of Salmonella (TA1537, TA1538, and TA98) both in the presence and absence of metabolic activation systems (59). The extracts were negative in strains TA100 and TA102. Aerosols generated by firing the M16 rifle were fractionated according to aerodynamic diameter. Sub‐micrometer particles were far more mutagenic than particles with aerodynamic diameters between 1 and 15 μm. The mutagens associated with the smaller particles were more active in the presence of S9, while extracts of larger particles were as active, or more active, in the absence of 59. Heavier particles, which settled rapidly out of the airstream, were not mutagenic.