The binding capacity of penicillin G-sulfoxide towards the penicillin-binding proteins (PBP) of Staphylococcus aureus H was studied. The sulfoxide and its parent compound, penicillin G, differ only in two aspects, the sulfur-bound oxygen and an altered conformation of the five-membered thiazolidine-ring system. These minor alterations of the penicillin structure resulted in a drastical decrease of binding activity (about two orders of magnitude) of the sulfoxide derivative towards its target enzymes. Furthermore, the sulfoxide did not exhibit the selectivity of subinhibitory doses for PBP 3, as could be observed for penicillin G. The biological consequences of this behaviour were monitored via growth curves, uptake of cell wall label, and analysis of the cell wall. Binding studies revealed that comparable growth inhibition and impairment of cell wall label uptake were achieved by at least a 100-fold higher penicillin G-sulfoxide concentration, compared to its parent compound. In cell wall analysis, the application of high doses of the antibiotics, i.e. nearly saturated PBP, verified the above mentioned observation. Surprisingly, small but significant differences in cell wall composition occurred using subinhibitory doses, probably due to the altered affinity towards PBP 3, supporting the hypothesis of an important role of this PBP in peptidoglycan transpeptidation.