During the 1979 workshop of the working group on ultrafine aerosols (WUFA) an intercomparison of different instruments for measurement of aerosol number concentrations was performed. Each of these instruments (TSI-aerosol electrometer, TSI-condensation nuclei counter, Jaenicke-condensation nuclei counter, size analyzing nuclei counter SANC) can be regarded as "absolute", because they do not depend on empirical calibration relative to external reference standards. Number concentrations were measured for monodispersed NaCl-aerosols with a mean particle diameter of 56 nm, generated by means of a collision atomizer and an electrostatic aerosol classifier. The transmission losses in the SANC-humidifier were determined quantitatively and the SANC-measurements were properly corrected. Measurements were performed over a concentration range from about 2.5 × 102 to 3.5 × 105 cm-3. Considering the differences between the various applied measuring techniques, fair agreement of the obtained concentration data was observed. The TSI-condensation nuclei counter agrees closely with the TSI-aerosol electrometer, however, it can be regarded as an "absolute" instrument only for concentrations below 103 cm-3. For low concentrations the Jaenicke-condensation nuclei counter tends to agree with the average concentration, obtained from all instruments used, above 104 cm-3 it agrees closely with the SANC. The readings of TSI-aerosol electrometer and SANC are quite linearly related over the whole concentration range, the SANC being low by a factor of about 0.59. Thus different measuring techniques, based on completely different principles, yield comparable aerosol number concentrations and accordingly condensation nuclei counters are truly aerosol counters.