Abstract
Potato spindle tuber viroid can be separated into two fractions by polyacrylamide gelelectrophoresis in the presence of formamide and urea. One fraction contains predominantly circular molecules; the second fraction contains almost exclusively linear molecules. The purity of the fractions was estimated by electron microscopy of formaldehyde-denatured molecules. The length distributions of denatured circular and linear molecules were determined. Both circular and linear molecules were found to be infectious.