The Finnish Meteorological Institute-Aerosol Cloud Interaction Tube (FMI-ACIT) is a multi-purpose instrument for investigating atmospherically relevant interactions between aerosol particles and water vapor under defined laboratory conditions. This work introduces an experimental setup of FMI-ACIT for investigation of the aerosol activation and the droplet growth under supersaturated conditions. Several simulations and experimental tests were conducted to find out what the proper operational parameters are. To verify the ability of FMI-ACIT to perform as a cloud condensation nuclei (CCN) counter, activation experiments were executed using size selected ammonium sulfate [(NH4)2SO4] particles in the size range of 10-300 nm. Supersaturations from 0.18% to 1.25% were tested by experiments with different temperature gradients. Those showed that FMI-ACIT can effectively measure CCN in this range. Measured droplet size distributions at supersaturations 0.18% and 1.25% are in good agreement with those determined by a droplet growth model.