Some, but not all, commercial preparations of micrococcal nuclease used to remove endogenous mRNA from reticulocyte lysates were found to contain proteolytic activity. The protease(s) caused a time‐dependent cleavage of the polyprotein primary translation products of genome RNA of several plant viruses, but did not affect the translation products of some other virus mRNA. The activity resulted in the production of smaller proteins and was inhibited by zinc ions. Thus the protease(s) mimicked virus‐coded proteases and represents a potential artifact in studies of translation products of virus RNA.