Gel electrophoretic analysis revealed marked alterations in the pattern of acid-extractable proteins from tomato leaves after infection with a viroid (PSTV), two viruses [tobacco mosaic virus (TMV) and cucumber mosaic virus (CMV)], and a fungus (Cladosporium fulvum) when compared to the pattern from healthy leaves. A pathogen-specific appearance of new protein bands was only found after infection with TMV (MW 17,400 and 65,000), CMV (MW 9000 and 8000) and Cladosporium fulvum (MW 28,000). With the exception of the TMV coat protein (MW 17,400) it could not be established whether the other four proteins are coded for by the corresponding pathogen or by the host. Nine proteins with the apparent NW of 10,000, 11,000, 12,000, 13,000, 14,000, 25,000, 31,000, 33,000 and 38,000 showed an increase in their relative concentration which is most dramatic in the case of the protein with the MW of 14,000 called p14. A decrease was observed in four proteins with molecular weights of 14,500, 23,000, 30,000 and 105,000. Since all these alterations could be correlated with the severity of the disease symptoms but not with the nature of the pathogen they must be considered as a general pathophysiological response of the tomato plant to infection and symptom development. A partial purification of the most prominent "pathogenesis-related" protein p14 from tomato plants is described.