The symptomatic effect of the amino acid type at residue 129 in the coat protein of cucumber mosaic virus was investigated in tobacco using coat protein mutants of the pepo strain in which proline 129 was substituted with 19 other amino acids. These mutants caused six types of symptoms: white mosaic, pale green mosaic, veinal chlorosis, veinal necrosis, systemic necrosis, and necrotic local lesions. Transmission electron microscopy revealed that the chloroplasts of plants showing the three former types of symptoms contained few thylakoid membranes. Cytopathic effects characteristic of cells from plants showing the three latter symptom types were not observed.