The objective of this study was to provide data for the design of multi-nozzle impactors giving size-fractionated samples for X-ray emission analysis and for size-distribution determinations of the alpha-particle activity from aerosol-particle-attached radon progenies. Different multi-nozzle impactors with laser-drilled 50-60 mu m diameter nozzles designed for a cut-off diameter of about 0.05 mu m were experimentally characterised. The nozzles were placed in a spiral pattern within a circle of 8.0 mm diameter. The nozzle-plate porosities were 0.013 and 0.050 (porosity = total cross-sectional area of nozzles divided by nozzle-plate area). The results show that laser-drilled nozzles can be used for multi-nozzle impactors and that the maximum nozzle-plate porosity is limited by the cross flow. The collection characteristics of the multi-nozzle impactors studied here were found to be more dependent on the Reynolds number and the S/W ratio [the ratio of the distance (S) between nozzle plate and impaction plate to the nozzle diameter (W)], than what has been shown for single-nozzle impactors. The S/W ratio and the Reynolds number must be considered if high size resolution is to be achieved.