We investigated the effect of a subminimal concentration of penicillin on the ultrastructure and protein profile of Group G streptococci. In cells treated with penicillin (1/3 MIC), the protein content increased by 50%, and several protein bands with a molecular mass of 14-70 kDa were detected. In the hydrophilic phase, carbohydrate-containing proteins were detected by PAS staining, and in the hydrophobic phase, a group of proteins that reacted strongly with homologous antisera were observed. In terms of cell structure, Triton X-114 extraction was found to induce alterations in the cross wall of untreated cells. In bacteria treated with penicillin but not extracted with Triton X-114, the cell wall was observed to detach itself, and regions with reduced amounts of cellular material appeared in the cytoplasm. After Triton-X114 extraction, these penicillin-treated cells exhibited profound morphological changes, leading in some cases to lysis.