All beta-lactam antibiotics exert their biological effects by interacting with a unique class of proteins, the penicillin-binding proteins (PBPs). These membrane proteins are involved in the biosynthesis of the murein or peptidoglycan, a mesh-like structure which completely surrounds the bacterial cell. Sequence similarities indicate that one domain of these proteins belongs to a large family of beta-lactam-recognizing proteins, which includes the active-site serine beta-lactamases. We here report the first three-dimensional crystal structure of a high molecular weight penicillin-binding protein, PBP2x of Streptococcus pneumoniae, at 3.5 A resolution. The molecule has three domains, the central domain being a transpeptidase, which is a suitable target for antibiotic development.