One technique used to measure concentrations of the hydroperoxy radical (HO$_2$) in the atmosphere involves chemically converting it to OH by addition of NO and subsequent detection of OH. However, some organic peroxy radicals (RO$_2$) can also be rapidly converted to HO2 (and subsequently OH) in the presence of NO, interfering with measurements of ambient HO$_2$ radical concentrations. This interference must be characterized for each instrument to determine to what extent various RO$_2$ radicals interfere with measurements of HO$_2$ and to assess the impact of this interference on past measurements. The efficiency of RO$_2$-to-HO$_2$ conversion for the Indiana University laser-induced fluorescence– fluorescence assay by gas expansion (IU-FAGE) instrument was measured for a variety of RO$_2$ radicals. Known quantities of OH and HO$_2$ radicals were produced from the photolysis of water vapor at 184.9 nm, and RO$_2$ radicals were produced by the reaction of several volatile organic compounds (VOCs) with OH. The conversion efficiency of RO$_2$ radicals to HO$_2$ was measured when NO was added to the sampling cell for conditions employed during several previous field campaigns. For these conditions, approximately 80 % of alkene-derived RO$_2$ radicals and 20 % of alkane-derived RO2 radicals were converted to HO$_2$. Based on these measurements, interferences from various RO$_2$ radicals contributed to approximately 35 % of the measured HO$_2$ signal during the Mexico City Metropolitan Area (MCMA) 2006 campaign (MCMA-2006), where the measured VOCs consisted of a mixture of saturated and unsaturated species. However, this interference can contribute more significantly to the measured HO$_2$ signal in forested environments dominated by unsaturated biogenic emissions such as isoprene.