The cell-wall composition and degree of cross-linking of peptidoglycan in a strain of Staphylococcus aureus (strain MR-1) which is highly resistant to methicillin were similar to those of other strains of S. aureus. When the organism was grown in the presence of very low concentrations of methicillin (equivalent to 3 x 10(-4) x minimum growth-inhibitory concentration [MGIC] there was a large decrease in the degree of cross-linking of the peptidoglycan. Increasing concentrations of methicillin (up to 1.25 x 10(-2) x MGIC) caused a further decrease in cross-linkage but thereafter a minimum value was reached. This remained unchanged even after growth of the organisms in much higher concentrations of the antibiotic up to 0.3 x minimum growth-inhibitory concentration. S. aureus MR-1 was able to grow normally for many generations under these conditions and reduction in cross-linkage of peptidoglycan was the only change detected in wall chemistry. Growth in the presence of methicillin (up to 0.3 x MGIC) (or other beta-lactam antibiotics) did not lead to an imbalance in the biosynthesis of peptidoglycan since no soluble polymers were secreted into the growth medium and nucleotide-linked precursors did not accumulate intracellularly. High concentrations of beta-lactam antibiotics (5 x MGIC) were bacteriostatic not bactericidal and this may be related to an apparent deficiency in the endogenous autolytic enzymes of strain MR-1. Studies of the penicillin-binding proteins after growth in the presence of methicillin suggest that one of these proteins remains resistant to very high concentrations of the antibiotic. We propose that this protein acts as the primary transpeptidase responsible for the incorporation of newly synthesised peptidoglycan into the growing wall.