Ultrastructural examination of tobacco mosaic virus-induced local lesions developing in leaves of Datura stramonium plants demonstrated that, in the central area of the lesions, the cell response to viral invasion was not uniform. Most cells exhibited an acute hypersensitive reaction and underwent rapid and complete necrosis. However, some cells, despite considerable virus accumulation and immediate contact with completely collapsed cells, maintained a certain degree of structural integrity. Analysis performed showed that the proportion of collapsed and uncollapsed cells in the lesion centre 3 to 5 days after infection did not change essentially. These data suggest that the absence of hypersensitive response in some cells in the lesion centre is not due to an early stage of infection but is likely caused by cell tolerance of the virus.