A viral probe was used in attempts to develop an in vivo test of the hypothesis that cellular senescence is accompanied by an increased rate of errors in macromolecular synthesis. Young and senescent Balb/cNNia mice were infected with encephalomyocarditis (EMC) virus. No differences in pattern of infection or titers of virus in brain and heart were observed between the two age groups. The yield of virus in control experiments was shown to be reduced by growth in the presence of 5-fluorouracil or a mixture of three amino acid analogs. Since the growth of this virus is highly dependent upon host cell synthetic machinery, these results are thought to suggest that substantial elevations in the rate of errors in macromolecular synthesis in these tissues do not occur with age. Further studies might allow a more precise determination of whether there is an age correlation of in vivo error rates; for the EMC virus, a selectable marker suitable for the quantitation of rates of mutation in vivo has so far not been obtainable.