Two independent modifications were made to one of two identical side-by-side filter banks for a large air-handler: ( 1) new filter holders, intended to provide a better seal and ( 2) ultraviolet germicidal irradiation (UVGI) lamps, intended to control microbiological growth. Total system efficiency testing with optical particle counters was performed to determine the effectiveness of the new filter holders. The efficacy of the UVGI lamps was determined through biological surface and air sampling. Results indicated that the side with the new filter holders was about 3.4% more efficient per year during the 22-month sampling period, whereas the coil chamber with the UVGI lamps had less biological contamination than the corresponding control chamber for over 75% of the sampling days.