The RNA-dependent RNA polymerase (RdRp) of plants has been proposed to act as core RNA polymerase in plant viral RNA replication. An antiserum directed against the host-encoded RdRp from cowpea leaves was applied to examine a possible role of this enzyme in cowpea mosaic virus (CPMV) RNA replication. The amount of host-encoded RdRp increased more than 20-fold in cowpea leaf cells infected with CPMV. In striking contrast, no increase of RdRp was found to accompany CPMV RNA replication in cowpea mesophyll protoplasts after infection with CPMV, indicating that an increased amount of RdRp is not a prerequisite for CPMV RNA replication. Furthermore, it is shown that the host-encoded RdRp is not present in the partially purified CPMV RNA replication complex, which is capable of synthesizing full-length viral RNAs. It is concluded that, contrary to a currently popular hypothesis, the host-encoded RdRp is not involved in viral RNA replication. The strong increase of RdRp seems to represent a tissue-related host response to virus infection, the physiological significance of which is not yet clear.