The peptide bond-forming reaction of protein synthesis, the peptidyl transfer reaction, takes place in a region of the 50S ribosomal subunit that consists entirely of RNA, the peptidyl transferase centre. Basic to the present knowledge of peptidyl transfer was the discovery by Robin Monro and his colleagues in the 1960s that the reaction is catalyzed by the 50S ribosome. The Monro experiments, and the historical context in which they were conceived, are described in this personal recollection. Monro's 'fragment reaction', the ribosome catalyzed reaction of a fragment of formylmethionyl-tRNA with puromycin, remains in use in work on peptidyl transfer.