This paper reports results from a field study designed to demonstrate a method for evaluating a building's in-duct ultraviolet germicidal irradiation (UVGI) system using naturally occurring airborne microorganisms and to evaluate its potential effectiveness against selected airborne microorganisms. Forty-eight 64 in. (1.63 m) long ultraviolet germicidal lamps were installed perpendicular to airflow and upstream of the cooling coils in a heating, ventilating, and air-conditioning (HVAC) system for a retail space in a large New York City office building. A portable in-duct UVGI apparatus was used to benchmark ultraviolet susceptibility of naturally occurring airborne microorganisms. Benchmarking was accomplished by measuring the portable apparatus's efficiency for inactivating UVGI-resistant microorganisms in the laboratory at various operating conditions. Inactivation efficiency for both the in-duct UVGI-equipped HVAC system and portable in-duct UVGI apparatus were then measured simultaneously using naturally occurring microorganisms. If the UVGI-equipped HVAC system had higher inactivation efficiency than the portable in-duct UVGI apparatus, then the UVGI-equipped HVAC system's efficiency for inactivating the UVGI-resistant microorganisms used in the laboratory would be expected to be higher than the efficiency of the portable in-duct UVGI apparatus. Based on the results for the tested UVGI-equipped HVAC system, the expected efficiency was at least 88% for Bacillus subtilis var. niger spores, a common surrogate for Bacillus anthracis spores.