SummaryGuanylation of tRNA by a lysate of rabbit reticulocytes was reported previously by Farkas and Singh. This reaction was investigated further using 18 purified E. coli tRNAs as acceptors. Results showed that only tRNA^Tyr, tRNA^His, tRNA^Asn and tRNA^AsP which contain the modified nucleoside Q in the anticodon acted as acceptors. Analysis of the nucleotide sequences in the guanylated tRNA showed that guanine specifically replaced Q base in these tRNAs.