The detection and quantification of biological particles in the atmosphere is becoming an increasingly important field of study. One instrument designed for this purpose, known as WIBS-4, can measure individual particles and their number-size distributions in two pre-defined size ranges: (i) similar to 0.3-12 mu m and (ii) similar to 3-31 mu m (optical diameter, polystyrene latex equivalent) while also making measurements of fluorescence across two wavebands. It can also supply simultaneous information on particle asymmetry. Since particle number concentrations are provided by the technique, the counting efficiency of WIBS-4 needs to be defined. Therefore in the current study, a method for calibration suitable and necessary for use with single particle measuring instruments is described. The lower-end counting efficiency curve for the WIBS-4 was thereby defined in specific size regimes for the first time (D-50 similar to 0.489 mu m and D-100 similar to 0.69 mu m). Experiments were conducted at room temperature and atmospheric pressure using a range of different sized PolyStyrene Latex (PSL) microspheres. A TSI 3010 condensation particle counter, using butanol as a working fluid, was used as a reference particle counter. The results also demonstrated a relationship between fluorescence intensity and the calculated volume of the PSL particles, when lambda(ex) =280 nm and lambda(em) similar to 310-400 nm. To our knowledge the current study represents the first to provide the fluorescence particle sensor community with both the counting efficiency of the WIBS-4 and a relationship plot giving a fluorescence intensity-particle size correlation over a broad PSL size range. (C) 2012 Elsevier Ltd. All rights reserved.