The surface characteristics of residues obtained from several commercial antibiotic products were studied using scanning electron microscopy. The photomicrographs showed the presence of particulates possessing crystalline or amorphous properties with particle sizes ranging from 1 mum to several mum. Small, well-defined, granular crystalline particulates seemed to predominate in residues of products which were processed under optimum pH conditions. Coalescing or aggregating of small crystalline particles (1-10mum) to form larger masses was interpreted as a sign of instability of the antibiotic. Most of the penicillin and semisynthetic penicillins produced residues that were characterized as amorphous, flaky, bulky and of no distinct shape. Such structures were attributed to the drying of gelatinous particulates. The possible sources and causes of the occurrence of these particulates are discussed. It appears that in many of the products the particulates are product-related rather than process-related, and subtle degradation may be involved. Advice is given to practitioners regarding the preparation, storage and administration of these products.