Pneumococci growing on choline-containing medium are known to incorporate this amino alcohol into the wall teichoic acid and produce autolysin-sensitive cell walls. In contrast, bacteria grown on the choline analogue, ethanolamine, incorporate ethanolamine into the teichoic acid and synthesize cell walls that are resistant to the homologous autolysin. In this communication, we report experiments aimed at understanding the biochemical mechanism of this phenomenon. Ethanolamine-containing (autolysin-resistant) cell walls were methylated in vitro with methyl iodide. Under appropriate conditions, virtually all of the ethanolamine residues could be converted to choline. After methylation, the formerly autolysin-resistant walls could be quantitatively hydrolyzed by the pneumococcal autolysin. Methylated walls also recovered another property typical of cell walls isolated from choline-grown bacteria: they could induce the in vitro "conversion" of an inactive form of autolysin to the catalytically active form (Tomasz, A., and Westphal, M. (1971) Proc. Natl. Acad. Sci. U.S.A. 68, 2627-2630). The results suggest that the autolysin-catalyzed hydrolysis of amide bonds in the peptidoglycan requires an additional interaction between the enzyme protein and choline residues in the teichoric acid portion of the cell wall.