This paper presents the results of the laboratory evaluation of the Ultraviolet Aerodynamic Particle Size Spectrometer, the novel instrument for real-time continues monitoring of bioaerosols. The main focus of this study was on evaluating selectivity, sensitivity, counting efficiency, and the detection limits of the UVAPS. The tests were performed with two types of aerosols, bacterial (e.g., Bacillus subtilis spores or vegetative cells, and Pseudomonas fluorescens) and non-bacterial (e.g., NaCl, latex, peptone water, and nutrient agar/broth). To control viability of airborne bacteria, the bioaerosols were simultaneously sampled with the AGI-30 impingers. The study has demonstrated the UVAPS cross-sensitivity to the non-bacterial organic materials and the need for careful preparation (washing) of test bacteria, in order to avoid the interference with the fluorescence signals of nutrient media used to grow bacteria. The results were indicative of strong sensitivity of the UVAPS to the type of airborne bacteria. The limitations in the capability of the UVAPS to measure bacterial spores were also found. Counting efficiency of the fluorescent particles was shown to depend on particle concentration with the upper limit of detection of the UVAPS approximately 6 x 10(7) particles/m(3). Crown Copyright (C) 2002 Published by Elsevier Science Ltd. All rights reserved.