The Bartley-Fox gun law, which became effective in April 1975, made the illicit carrying of a firearm punishable with a 1-year mandatory prison term. The legislation was targeted against those persons who carried guns as a matter of lifestyle without specific criminal purposes in mind and was highly publicized for 2 months prior to its implementation. This study examined the issues of deterrence and weapon substitution by using interrupted time series techniques to assess changes in assault levels between 1974, 1975, and 1976 and comparing crime rate changes in Boston with selected control jurisdictions. The law's effect on citizen reporting was also explored. Analyses of data from Boston and other areas in Massachusetts showed that the law substantially reduced the incidence of gun assaults, but produced a more than offsetting increase in nongun armed assaults. Evidently, the statute prevented some individuals from carrying and using their firearms, but did not stop them from becoming involved in assaultive situations and resorting to other weapons. In Boston, the law appeared to have increased the likelihood of citizens reporting less serious forms of gun assaults to the police, although this phenomenon tended to obscure the law's deterrent effects. The Bartley-Fox statute resulted in a reduction in gun robberies, accompanied by a less than corresponding increase in nongun armed robberies. Finally, the law reduced gun homicides with no increase in nongun homicides. In the assault analysis where the effects were most pronounced, the decline in gun assaults in Boston started 1 month prior to the effective date of the law. This suggests that the gun law may have achieved its impact primarily through its announced intent rather than actual implementation. Tables and 26 footnotes are included. (Author summary modified)