Ambient HO measurements by low-pressure laser-excited fluorescence with chemical modulation, and supporting ozone and water-vapor data, are presented for periods in May and August 1987. The observed peak daytime ambient HO concentrations are in the range (2.5 to 8)¿106 molecules cm-3 and show small negative offsets due to photochemical interference. Direct measurements of the interference at fixed 3> give the dependence on ambient 2O> and on the modulating reagent . At ambient 3>=30 ppb and 10 torr H2O, with excitation and detection at a total pressure of 4 torr, the net interference is equal to =-1.3¿106 molecule cm-3. Production of HO by the reaction of isobutane with O(1D) accounts for the negative interference. Quenching of HO fluorescence by the modulating reagent contributes a smaller positive term to the interference; kinetic measurements of the quenching rate coefficient yield kQ0i(HO A (v'=0)+isobutane)=(1.0¿0.15)¿10-9 cm3 molecule-1 s-1. The experimental interference results are compared with a detailed kinetic model of HO production, excitation, relaxation, and detection; reasonable agreement is found despite uncertainties in beam spatial and temporal profiles and in the rate coefficients necessary to the model. The model also computes the interference due to H2O2 photolysis. Net interference and signal-to-noise ratio are computed as functions of photon flux for 1,0 (YAG/dye 282 nm) and 0,0 (Cu/dye 308 nm) excitation. ¿ American Geophysical Union 1992