The moisture content changes under optimum drying conditions of a droplet of alpha-amylase solution leaving the nozzle in a specially designed horizontal spray dryer was investigated. In a horizontal spray dryer of 6 x 3 x 3 m size, the gas entered at 155 degrees C concurrently with the alpha-amylase solution being sprayed from two pressure nozzles. The particles were collected in a channel through which they were transported to the cyclone pneumatically. The air rate was adjusted to keep the temperature below 60 degrees C during the powder travel in the channel. During the operation, the activity loss of the powder was determined as 9%. The fully dry, equilibrium and critical moisture contents of the powder were determined by off-line experiments. These values were fed to the computer program to predict the final moisture content of particles. A good agreement between the predicted and tile experimental values was found.