4-Chlorobiphenyl (4-PCB) and Arochlor 1221 have been studied in the presence of cetylpyridinium bromide micellar medium. The quenching produced by this surfactant on the fluorescence of these organochlorine compounds has been used to develop two spectrofluorimetric methods for the sensitive determination of 4-PCB and Arochlor 1221. Variables such as the percentage of organic solvent, temperature, and concentration of surfactant, as well as the possible interferences of other polychlorinated biphenyls and Arochlors of environmental and toxicological interest have been studied. The methods have been applied to the determination of 4-PCB and Arochlor 1221 added to sea water with satisfactory recovery data.