A rapid and simple protocol for the isolation of RNA from transfected tissue culture cells is described. The protocol employs a guanidinium thiocyanate/phenol mixture to lyse cells directly from tissue culture plates and extract the total RNA. A total of six simple steps, which can be accomplished within 2.5 hours, are required. The protocol reproducibly yields 20-40 micrograms RNA from 0.5 x 10(6) - 1 x 10(6) cells per sample. The quality of the RNA obtained is sufficient for reverse transcriptase assays such as oligonucleotide-directed primer extension and random-primed cDNA synthesis.