Recently, there has been much progress in understanding tRNA identity, i.e. in elucidating the sets of nucleotides that are responsible for the specific aminoacylation of a tRNA with its cognate amino acid. Interest focused, however, on tRNAs from Escherichia coli and yeast. Here we have identified the major and minor determinants of human tRNA(Ser) which were revealed by an identity switch from human tRNA(Val) to tRNA(Ser). We used in vitro transcripts and subsequent aminoacylation by HeLa S100 extract to determine the kinetic parameter Vmax/Km. The two major identity elements which are absolutely required for aminoacylation by human seryl-tRNA synthetase are the discriminator base and the long extra arm. This is in contrast to E. coli tRNA(Ser) where the discriminator base is unimportant, whereas identity determinants in the acceptor stem are required. Other sequence elements have an influence not only on serylation, but also on tRNA maturation in vitro, i.e. on pre-tRNA processing and base modification. These nucleotides are located in the DHU and the T phi C arm and are probably necessary for the proper folding of tRNAs containing a long extra arm. A34 to inosine modification depends highly on the correct three-dimensional structure of the tRNA, whereas A58 to m1A methylation does not rely on the three-dimensional folding of the substrate. This is the first tRNA identity switch involving the exchange of a short versus a long extra arm.