Microparticles of ampicillin have been precipitated by a supercritical antisolvent process (SAS) using carbon dioxide and N-methylpyrrolidone as the antisolvent and solvent, respectively. The mean particle size (PS) and particle size distribution (PSD) of the processed antibiotic were chosen as responses to evaluate the process performance. The levels for a screening design of experiments were chosen to allow the process to take place in a single supercritical phase. Under these conditions, all of the, experiments led to the successful precipitation of ampicillin. Concentration was a key factor as this had the most marked effect on; both PS and PSD.