Laboratory comparisons have been made of aerosol concentrations indicated by four different types of condensation nucleus counters. Three of these counters, the Langer, Rosen, and General Electric SANDS instruments have been used to measure Aitken nuclei concentrations in the upper troposphere and the stratosphere, and the fourth, a Pollak counter, had been carefully calibrated to serve as a standard. Except for the smallest particles employed, quite good agreement was experienced among the Rosen, SANDS and Pollak counters, and the tests served to calibrate the Langer instrument.