Two competing events, termination and readthrough (or nonsense suppression), can occur when a stop codon reaches the A-site of a translating ribosome. Translation termination results in hydrolysis of the final peptidyl-tRNA bond and release of the completed nascent polypeptide. Alternatively, readthrough, in which the stop codon is erroneously decoded by a suppressor or near cognate transfer RNA (tRNA), results in translation past the stop codon and production of a protein with a C-terminal extension. The relative frequency of termination versus readthrough is determined by parameters such as the stop codon nucleotide context, the activities of termination factors and the abundance of suppressor tRNAs. Using a sensitive and versatile readthrough assay in conjunction with RNA interference technology, we assessed the effects of depleting eukaryotic releases factors 1 and 3 (eRF1 and eRF3) on the termination reaction in human cell lines. Consistent with the established role of eRF1 in triggering peptidyl-tRNA hydrolysis, we found that depletion of eRF1 enhances readthrough at all three stop codons in 293 cells and HeLa cells. The role of eRF3 in eukarytotic translation termination is less well understood as its overexpression has been shown to have anti-suppressor effects in yeast but not mammalian systems. We found that depletion of eRF3 has little or no effect on readthrough in 293 cells but does increase readthrough at all three stop codons in HeLa cells. These results support a direct role for eRF3 in translation termination in higher eukaryotes and also highlight the potential for differences in the abundance or activity of termination factors to modulate the balance of termination to readthrough reactions in a cell-type-specific manner.