Summary The activity of acid nuclease was investigated in healthy and potato spindle tuber viroid (PSTV)-infected tomato leaves. The enzyme has some characteristics of nuclease I, exhibiting DNase, RNase and presumably 3′-nucleotidase activities. It also showed preference for heat-denatured DNA versus native DNA and was active at pH 5.2. Molecular sieving of the extract from infected tomato leaves produced one distinct nuclease peak with an apparent molecular mass of 26 kDaltons. The nuclease extracted from healthy and PSTV-infected leaves exhibited similar PAGE patterns under both native and denatured conditions. On the other hand, some quantitative and qualitative differences were observed on SDS-PAGE and from immunoelectrophoresis patterns of proteins extracted at pH 5.2 from infected and healthy leaves. Both, circular and linear forms of PSTV were found to be cleavable with the nuclease. But PSTVspecific RNA was about 900-fold more resistant substrate to cleavage with plant nucleases than highly polymerized RNA from yeast.