Determinations were made of the rate of growth of a drop of water supported by an upward flow of a cloud whose free water content and droplet-radius distribution were measured. The water content was varied through a range of 5.5 to 10.9 grams per cubic meter, but the droplet-radius distribution was kept constant with a single maximum at a radius of 5.2 microns and with no radii exceeding 18µ. Collection efficiencies are evaluated directly from the data and are compared to those calculated from Langmuir's theory. The measured values of the efficiencies are found to be significantly greater than the theoretical values when the radius of the growing drop lies within the range from 150µ to 0.5 mm. When the radius of the drop lies outside this range, the measured efficiency is much smaller than theory predicts.