Supercritical antisolvent precipitation (SAS) has been used to produce micronized particles of some antibiotics. Griseofulvin, ampicillin, amoxicillin, and tetracycline have been tested in N-methylpyrrolidone, dimethylsulfoxide, ethyl alcohol and methylene chloride. Some coupled antibiotic-liquid solvent have been successfully processed. Amorphous particles have been obtained at all successful SAS conditions while crystals have been sometimes observed when the micronization process failed. For example, spherical nanoparticles have been observed for tetracycline in N-methylpyrrolidone. These particles tend to coalesce in small aggregate-forming groups. The effect of the SAS process parameters on morphology, particle size and particle size distribution have been studied for the system tetracycline / N-methylpyrrolidone. The coalescence of tetracycline particles decreases with the increase in operating pressure; whereas the diameter of the aggregates increases with tetracycline concentration in the liquid solvent. Operating at 180 bar, 40 degrees C and at different concentrations, aggregates with mean diameters between 0.6 and 0.8 mu m have been observed.