Sequence comparison of the tomato ringspot nepovirus (TomRSV) genome with related viruses suggested that the region of the RNA-2-encoded polyprotein immediately upstream of the coat protein may be involved in the cell-to-cell movement of the virus (Rott et al., 1991, J. Gen. Virol. 72, 1505-1514). To further study the role of this portion of the genome, monoclonal antibodies against the putative movement protein were raised. Western blots of plant extracts allowed the detection of a viral nonstructural protein of M(r) 45K present only in TomRSV-infected tissues. Immunogold-labeling studies revealed that in Nicotiana clevelandii the putative movement protein was found only in infected cells immediately adjacent to the necrotic tissue, and that it was associated with tubular structures containing virus-like particles present in or near the cell wall. This provides further evidence that this protein is involved in the cell-to-cell movement of the virus and that this movement might take place via the formation of tubular structures.