The development of therapeutic bispecific T cell-engaging antibodies, which recruit cytotoxic T cells to tumour cells, has been hampered by manufacturing challenges as well as their short serum half-life. To circumvent these issues, Stadler et al. generated 1-methylpseudouridine-containing mRNAs encoding bispecific antibodies directed against the T cell receptor-associated molecule CD3 and a tumour-associated antigen. In mice, injected antibody-encoding mRNAs achieved sustained therapeutic antibody levels and safely eliminated tumours in human ovarian carcinoma xenograft models.